Method and apparatus for transceiving reference signal in wireless communication system

ABSTRACT

Disclosed are a method and an apparatus for transceiving a reference signal in a wireless communication system. The present invention discloses a scheme for transceiving the reference signal the reference signal that is selectively added based on the capacity and characteristics of a terminal. The added reference signal may include newly defined CRS, DM-RS, TRS, MBSFN-RS or new RS or the like for cell recognition of the terminal, synchronization signal, measurement, etc. The location of a specific symbol/frequency of the like and selective power boosting are also disclosed for the added reference signal. Further, the transmission density of RS can be adaptively and selectively channel and transmitted as needed. Thus, data can be adaptively transceived in consideration of the coverage based on the location of characteristics of the terminal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communication and, moreparticularly, to a method and an apparatus for transceiving a referencesignal in a wireless communication system.

2. Related Art

Recently, the necessity to easily obtain and transfer information anddata as necessary by connecting peripheral things (objects) via anetwork, without limitations in a location or a time, has increased. Inline with this, machine-to-machine/Internet of things (IoT) allowing forthe provision and use of various services according to user demand hasemerged as a major issue for next-generation communication market.Initial M2M started from a sensor and radio frequency identification(RFID) network targeting a partial area. Recently, however, an interestin M2M based on a mobile communication network in consideration ofmobility of things, an extensive service area including the sea s wellas islands and mountain regions, ease in network operation andmaintenance, security for data transmission with high reliability, andguarantee of quality of service (QoS) tend to increase. The 3GPP, amobile communication standardization group, which started to researchM2M in 2005, has conducted a full-fledged standardization under thetitle of machine type communications (MTC). Here, machine refers to anentity not requiring human beings' direct manipulation or interventionand MTC refers to data communication including one or more machines.

For example, a smart meter equipped with a mobile communication module,a vending machine, and the like, may be included in MTC, and recently, asmartphone, or the like, automatically accessing a network to performcommunication without user manipulation and intervention according touser locations or situations has been considered as a mobile terminalmachine having the MTC function. In addition, an MTC device such as anIEEE 802.15 WPAN (wireless personal area network)-based micro-sensor ora gateway connected to RFID, or the like, has also been considered.

Thus, in order to accommodate numerous MTC devices transmitting andreceiving small amounts of data, an existing mobile communicationnetwork requires different identifiers and address systems used in anexisting communication system, and in a communication environment inwhich a plurality of communication devices coexist, a novelcommunication mechanism in consideration of a more effectivecommunication scheme and cost is required.

SUMMARY OF THE INVENTION

The present invention provides a method for a method and an apparatusfor transceiving a reference signal in a wireless communication system.

The present invention provides a method for a method and an apparatusfor transceiving a reference signal through changeable parametersaccording to coverage of a terminal in a wireless communication system.

To accomplish the above object, according to an aspect of the presentinvention, there is a provided a method for transceiving a referencesignal by a terminal in a wireless communication system, the methodincluding: determining capacity information of the terminal; confirminginformation on an added reference signal corresponding to the capacityinformation; and transceiving a corresponding reference signal at apredetermined symbol or a frequency domain using the information on anadded reference signal.

The added reference signal comprises at least one of a cell specificreference signal (CRS), a demodulation reference signal (DM-RS) or anMBSFN-RS, and the capacity information of the terminal comprises acoverage improvement request level confirming an intensity of a downlinksignal by the terminal and selected corresponding to the intensity ofthe signal, or information indicating that the terminal is a machinetype communication (MTC) terminal.

To accomplish another object, according to another aspect of the presentinvention, there is a provided an apparatus for transceiving a referencesignal in a wireless communication system, the apparatus including: aRadio Frequency (RF) unit that sends and receives signals to and from abase station; and a processor coupled to the RF unit to process thesignal.

The processor transmits capacity information of a terminal to the basestation, confirms information on an added reference signal correspondingto the capacity information, and transceives a corresponding referencesignal at a predetermined symbol or a frequency domain to and from abase station using the information on an added reference signal, and theprocessor sets to allow the capacity information of the terminal tocomprise a coverage improvement request level confirming an intensity ofa downlink signal by the terminal and selected corresponding to theintensity of the signal, or information indicating that the terminal isa machine type communication (MTC) terminal, and confirms that the addedreference signal include at least one of newly defined CRS, ademodulation (DM) reference signal, or a MBSFN-RS.

According to the present invention, a scheme of adaptively transmittingand receiving data according to coverage of a terminal in a wirelesscommunication system having variable coverage according to capability ofthe terminal and/or a location of the terminal. Accordingly, efficiencyof data transmission in the entire system can be enhanced. Also, sincedata is adaptively transmitted and received according to capability of aterminal in a communication environment mixed with a legacy terminal,limited radio resource may be effectively used. Thus, system efficiencyof the overall system can be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a structure of a radio frame to which the presentinvention is applied.

FIG. 2 is a view illustrating an example of a resource grid regarding adownlink slot, to which the present invention is applied.

FIG. 3 is a view illustrating a structure of a downlink subframe towhich the present invention is applied.

FIG. 4 is a view illustrating a structure of an uplink subframe, towhich the present invention is applied.

FIG. 5 is a view illustrating a structure of a radio frame in a TDDmodule, to which the present invention is applied.

FIG. 6 is a block diagram illustrating a method of generating PDCCHdata, to which the present invention is applied.

FIG. 7 is a view illustrating monitoring of a PDCCH, to which thepresent invention is applied.

FIG. 8 is a view illustrating a downlink subframe to which a referencesignal and a control channel are allocated, to which the presentinvention is applied.

FIG. 9 is a view illustrating an example of a subframe having an EPDCCH,to which the present invention is applied.

FIG. 10 is a conceptual view illustrating carrier aggregation (CA) towhich the present invention is applied.

FIG. 11 is a conceptual view illustrating Pcell and Scell, to which thepresent invention is applied.

FIG. 12 is a conceptual view illustrating a method of transmitting dataon the basis of coordinated multi-point (CoMP), to which the presentinvention is applied.

FIG. 13 is a view illustrating transmission of a synchronization signaland PBCH data in a legacy subframe when a frequency division duplex(FDD) is used in a duplex manner, to which the present invention isapplied.

FIG. 14 is a conceptual view illustrating transmission of a CSI-RS andfeedback of CSI measured in a terminal, to which the present inventionis applied.

FIG. 15 is a conceptual view illustrating a method of processing adownlink transport channel, to which the present invention is applied.

FIG. 16 is a conceptual view illustrating a PRACH used for random accessof a terminal, to which the present invention is applied.

FIG. 17 is a conceptual view illustrating a PRACH to which the presentinvention is applied.

FIG. 18 is a conceptual view illustrating a method of configuring a CRSaccording to an embodiment of the present invention.

FIG. 19 is a conceptual view illustrating a method of configuring a CRSaccording to an embodiment of the present invention.

FIG. 20 is a conceptual view illustrating a method of transmitting andreceiving data by a coverage-limited terminal according to an embodimentof the present invention.

FIG. 21 is a conceptual view illustrating a method of transmitting aPBCH according to an embodiment of the present invention.

FIG. 22 is a conceptual view illustrating a method of transmitting aPBCH according to an embodiment of the present invention.

FIG. 23 is a conceptual view illustrating a method of transmitting aPBCH using a circular repetition method according to an embodiment ofthe present invention.

FIG. 24 is a conceptual view illustrating a method of transmitting aPBCH using simple repetition according to an embodiment of the presentinvention.

FIG. 25 is a block diagram illustrating a wireless communication systemaccording to an embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

A wireless device may be fixed or mobile and may be referred to by othernames such as user equipment (UE), mobile station (MS), mobile terminal(MT), user terminal (UT), subscriber station (SS), personal digitalassistant (PDA), wireless modem, handheld device, terminal, wirelessterminal, and the like. The wireless device may be a device supportingonly data communication like a machine-type-communication (MTC).

A base station generally refers to a fixed station that communicateswith a wireless device and may be referred to by other names such asevolved-node B (eNB), base transceiver system (BTS), access point (AP),and the like.

Hereinafter, operations of a terminal and/or a base station in the 3GPPLTE (long term evolution), or 3GPP LTE-A (advanced) defined on the basisof each release of 3GPP (3rd Generation Partnership Project) TS(Technical Specification). Also, the present invention may also beapplied to various wireless communication networks, rather than to the3GPP LTE/3GPP LTE-A.

FIG. 1 shows the structure of a radio frame in 3GPP LTE.

Referring to FIG. 1, the radio frame includes 10 subframes 120, and onesubframe includes two slots 140. The radio frame may be indexed based onslot 140, that is, from slot #0 to #19 or may be indexed based onsubframe 120, that is, from subframe #0 to subframe #9. For example,subframe #0 may include slot #0 and slot #1.

A time taken for transmitting one subframe 120 is called a transmissiontime interval (TTI). The TTI may be a scheduling basis for a datatransmission. For example, a radio frame may have a length of 10 ms, asubframe may have a length of 1 ms, and a slot may have a length of 0.5ms.

One slot 140 includes a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in a time domain and a plurality ofsubcarriers in a frequency domain. In LTE, a BS uses OFDMA as an accessmethod in downlink channel. The OFDM symbols are used to express asymbol period, and may be called by other names depending on amultiple-access scheme. For example, in an uplink channel in which awireless device transmits data to a BS, a single carrier-frequencydivision multiple access (SC-FDMA) may be used. The symbol section inwhich data is transmitted through uplink channel may be referred to as aSC-FDMA symbol.

The structure of radio frame 100 introduced in FIG. 1 is an embodimentfor the frame structure. Accordingly, new radio frame format may bedefined by changing the number of subframes 120, the number of slots 140included in the subframe 120, or the number of OFDM symbols included inthe slot 140.

In the radio frame structure, the number of symbols included in a slotmay be changed depending on which cyclic prefix (CP) is used. Forexample, when the radio frame uses a normal CP, one slot may includeseven OFDM symbols. When the radio frame uses an extended CP, one slotmay include six OFDM symbols.

The wireless communication system may be divided into a frequencydivision duplex (FDD) scheme and a time division duplex (TDD) scheme.According to the FDD scheme, an uplink transmission and a downlinktransmission may be performed based on different frequency bands.According to the TDD scheme, an uplink transmission and a downlinktransmission may be performed based on the same frequency band by usingtime division scheme. A channel response of the TDD scheme issubstantially reciprocal since it uses the same frequency band. That is,in TDD scheme, a downlink channel response and an uplink channelresponse are almost the same in a given frequency band. Thus, theTDD-based wireless communication system may obtain the channel stateinformation from the channel state information of uplink channel. In theTDD scheme, the entire frequency band is time-divided for uplink anddownlink transmissions, so a downlink transmission by the BS and anuplink transmission by the wireless device cannot be simultaneouslyperformed.

FIG. 2 is a view illustrating an example of a resource grid for adownlink slot.

The downlink slot includes multiple OFDM symbols in a time domain, andincludes NRB resource blocks in a frequency domain. NRB as a number of aresource block within the downlink slot is determined depending ondownlink transmission bandwidth configured at a cell. For example, In aLTE system, NRB may be a value of 6 to 110 according to transmissionbandwidth in use. A resource block 200 may include a plurality ofsubcarriers in the frequency domain. An uplink slot may have a structuresame as that of the downlink slot.

Each element on the resource grid is referred to as a resource element200. The resource element 220 on the resource grid can be identified byan index pair (k, l). Here, k (k=0, . . . , N_(RB)×12-1) is the index ofthe subcarrier in the frequency domain, and l (l=0, . . . , 6) is theindices of the OFDM symbols in the time domain.

Here, one resource block 200 may include 7 OFDM symbols in the timedomain and 7×12 resource elements 220 composed of 12 subcarriers in thefrequency domain. Such size is just an example, and it is possible thatthe number of subcarriers and OFDM symbols constructing one resourceblock 200 varies. The resource block pair indicate a resource unitincluding two resource blocks.

The number of the OFDM symbols included in one slot may vary dependingon CP as mentioned above. In addition, the number of the resource blockincluded in one slot may vary according to the size of the entirefrequency bandwidth.

FIG. 3 is a view illustrating the structure of a downlink subframe.

The downlink subframe may be identified by two slots 310, 320 based ontime. Each slot 310 or 320 includes 7 OFDM symbols in a normal CP. Aresource region corresponding to 3 OFDM symbols (maximum 4 OFDM symbolsfor 1.4 MHz bandwidth), which arrive first, in the first slot may beused as a control region 350. Remaining OFDM symbols may be used as adata region 360 to which a traffic channel such as a physical downlinkshared channel (PDSCH) is assigned.

PDCCH, for example, may be the control channel for transmittinginformation on resource allocation and a transmit format in adownlink-shared channel (DL-SCH), uplink shared channel (UL-SCH)resource allocation, information on paging on PCH, information on asystem on the DL-SCH, and information on resource allocation for upperlayer control messages such as random access response over the PDSCH, atransmit power control command set for individual UEs within a random UEgroup and voice over internet protocol (VoIP) activation. Multiple unitsfor transmitting PDCCH data may be defined within the control region350. A UE may monitor a plurality of units for transmitting PDCCH datato obtain control data. For example, PDCCH data may be transmitted tothe UE based on an aggregation of one or more continuous control channelelements (CCE). The CCE may be one unit for transmitting PDCCH data. TheCCE may include a plurality of resource element groups. The resourceelement group is a resource unit including available 4 resourceelements.

A base station determines a PDCCH format based on downlink controlinformation (DCI), and attaches a cyclic redundancy check (CRC) to thecontrol information. The CRC is masked with a unique identifier(referred to as a radio network temporary identifier (RNTI)) accordingto an owner or a usage. If PDCCH is for a specific UE, a uniqueidentifier of the UE, e.g., C-RNTI (cell-RNTI), may be masked to theCRC. IF PDCCH is for a paging message, an identifier indicating paging,e.g., P-RNTI (paging-RNTI), may be masked to the CRC. If PDCCH is for asystem information block (SIB), a system information-RNTI (SI-RNTI)) maybe masked to the CRC. In order to indicate random access response asresponse for a random access preamble of a UE, a random access-RNTI maybe masked to the CRC.

FIG. 4 is a view illustrating a structure of an uplink subframe.

An uplink subframe may be divided into control regions 430 and 440 and adata region 450 with respect to a frequency domain. A physical uplinkcontrol channel (PUCCH) for transmitting uplink control information isallocated to the control regions 430 and 440. A physical uplink sharedchannel (PUSCH) for transmitting data is allocated to the data region450. When indicated in a higher layer, a terminal may supportsimultaneously transmission of the PUSCH and the PUCCH.

A PUCCH for a single terminal may be allocated in units of RB pairs in asubframe 400. Resource blocks belonging to the resource block pairs maybe allocated to different subcarriers in each of a first slot 410 and asecond slot 420. A frequency occupied by resource blocks belonging to aresource block pair allocated to the PUCCH is changed with respect to aslot boundary. Such a PUCCH allocation method is called a frequencyhopping method. A terminal may transmit different subcarriers over timeto obtain a frequency diversity gain. m is a position index indicating alogical frequency region position of the resource block pair allocatedto the PUCCH within a subframe.

Uplink control information transmitted on a PUCCH may include HARQ(hybrid automatic repeat request) ACK (acknowledgement)/NACK(non-acknowledgement), a CQI (channel quality indicator) indicating adownlink channel state, and SR (scheduling request) as an uplink radioresource allocation request.

A PUSCH is a channel mapped to UL-SCH (uplink shared channel) as atransport channel. Uplink data transmitted on a PUSCH may be a transportblock as a data block of a UL-SCH transmitted during a TTI. Thetransport block may include user information. Also, control informationmultiplexed to data may include CQI, PMI (precoding matrix indicator),HARQ, RI (rank indicator), and the like. Also, uplink data may includeonly control information.

FIG. 5 shows a downlink radio frame structure in TDD mode.

Referring to FIG. 5, a subframe having an index #1 and an index #6 iscalled a special subframe, and includes a downlink pilot time slot(DwPTS), a guard period (GP), and an uplink pilot time slot (UpPTS). TheDwPTS is used in the UE for initial cell search, synchronization, orchannel estimation. The UpPTS is used in the BS for channel estimationand uplink transmission synchronization of the UE. The GP is a periodfor removing interference which occurs in an uplink due to a multi-pathdelay of a downlink signal between the uplink and downlink.

In TDD, a downlink (DL) subframe and an uplink (UL) subframe co-exist inone radio frame. Table 1 shows an example of a configuration of theradio frame.

TABLE 1 Uplink- downlink config- Switch-point Subframe number urationperiodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U U D S U U U 1 5 ms D S UU D D S U U D 2 5 ms D S U D D D S U D D 3 10 ms  D S U U U D D D D D 410 ms  D S U U D D D D D D 5 10 ms  D S U D D D D D D D 6 5 ms D S U U UD S U U D

‘D’ denotes a DL subframe, ‘U’ denotes a UL subframe, and ‘S’ denotes aspecial subframe. When the UL-DL configuration is received from the BS,the UE can know whether a specific subframe is the DL subframe or the ULsubframe according to the configuration of the radio frame.

A DL (downlink) is divided into a control region and a data region in atime domain. The control region includes a maximum of first three OFDMsymbols of a first slot in a subframe, but the number of OFDM symbolsincluded in the control region may be changed. A PDCCH and other controlchannel are allocated to the control region, and a PDSCH is allocated tothe data region.

FIG. 6 is a block diagram showing a method for generating the PDCCHdata. FIG. 6 introduces a method for generating the PDCCH data indetail.

Refer to FIG. 6, a wireless device performs blind decoding for PDCCHdetection. The blind decoding may be performed based on an identifierwhich is masked from a cyclic redundancy check (CRC) of a received PDCCH(referred to as a candidate PDCCH). The wireless device may determinewhether the received PDCCH data are its own control data by performingCRC error checking on the received PDCCH data.

A BS determines a PDCCH format according to a downlink controlinformation (DCI) to be transmitted to a wireless device, attaches acyclic redundancy check (CRC) to the DCI, and masks a unique identifier(referred to as a radio network temporary identifier (RNTI)) to the CRCaccording to an owner or usage of the PDCCH (block 610).

If the PDCCH is for a specific wireless device, the BS may mask a uniqueidentifier of the wireless device, e.g., cell-RNTI (C-RNTI) to the CRC.Alternatively, if the PDCCH is for a paging message, the BS may mask apaging indication identifier, e.g., paging-RNTI (P-RNTI) to the CRC. Ifthe PDCCH is for system information, the BS may mask a systeminformation identifier, e.g., system information-RNTI (SI-RNTI) to theCRC. In addition, in order to indicate a random access response that isa response for transmission of a random access preamble, the BS may maska random access-RNTI (RA-RNTI) to the CRC, and in order to indicate atransmit power control (TPC) command for a plurality of wirelessdevices, the BS may mask a TPC-RNTI to the CRC.

The PDCCH which is masked by the C-RNTI carries control information fora specific wireless device (such information is called UE-specificcontrol information), and the PDCCH masked by other RNTIs may carrycommon control information received by all or a plurality of wirelessdevices in a cell. A plurality of DCI formats can be defined to transmitthe PDCCH data. This will be additionally described below.

The BS generates coded data by encoding the CRC-attached DCI (block620). The encoding includes channel encoding and rate matching.

The BS generates modulation symbols by modulating the coded data (block630).

The BS maps the coded data to physical resource elements (REs) (block640). The BS may map the modulation symbols to each resource element(RE).

As described above, the control region in a subframe includes aplurality of control channel elements (CCEs). The CCE is a logicalallocation basis used for providing the PDCCH with a coding ratedepending on a radio channel state, and corresponds to a plurality ofresource element groups (REGs). The REG includes a plurality of resourceelements. One REG includes four Res, andone CCE includes nine REGs. Inorder to configure one PDCCH, 1, 2, 4 or 8 CCEs may be used, and the CCEaggregated as a basis of 1, 2, 4 or 8 is referred to as a CCEaggregation level.

The BS determines the number of CCEs used in transmission of the PDCCHaccording to a channel state. For example, a wireless device having agood downlink channel state may use one CCE in PDCCH transmission. Onthe other hand, a wireless device having a poor downlink channel statemay use 8 CCEs in PDCCH transmission.

A control channel consisting of one or more CCEs performs interleavingon an REG basis, and may be mapped to a physical resource afterperforming cyclic shift based on a cell identifier (ID).

FIG. 7 is a view illustrating an example of monitoring PDCCH.

Referring to FIG. 7, an UE can perform blind decoding for detecting thePDCCH. Blind decoding is a scheme in which a desired identifier isdemasked to the CRC of a received PDCCH (referred to as a candidatePDCCH) and CRC error check is conducted so as to identify whether thecorresponding PDCCH is its own control channel. The terminal is notaware of CCE aggregation level or DCI format for transmission and aposition at which its PDCCH data is transmitted in a control region.

A plurality of PDCCHs may be transmitted in one sub-frame. The UEmonitors a plurality of PDCCHs at every sub-frame. Here, the term“monitoring” refers to the UE attempting to perform blind decoding on aPDCCH.

In 3GPP LTE, the UE uses a search space (SS) for reducing load caused byblind decoding. The search space may be regarded as CCEs' monitoring setfor searching a PDCCH. The UE monitors the PDCCH based on the searchspace.

The search space is divided into a common search space (CSS) and aUE-specific search space (USS). The common search space is a space forsearching a PDCCH having common control information and consists of 16CCEs, CCE index 0 to 15, and supports PDCCHs having a CCE aggregationlevel of {4, 8}. However, a PDCCH (DCI formats 0 and 1A) for carryingUE-specific information may be transmitted even in the common searchspace. The UE-specific search space supports PDCCHs having a CCEaggregation level of {1, 2, 4, 8}.

The following table 2 shows the number of PDCCH candidates that aremonitored by the UE.

TABLE 2 Search space S_(k) ^((L)) Number of Aggregation Size PDCCH DCIType level L [in CCEs] candidates M^((L)) Format UE-specific 1 6 6 0, 1,1A, 2 12 6 1B, 1D, 4 8 2 2, 2A 8 16 2 Common 4 16 4 0, 1A, 8 16 2 1C,3/3A

The size of a search space is determined according to Table 2 above, andthe start point of a search space is defined differently for each of thecommon search space and UE-specific search space. The start point of thecommon search space is fixed regardless of any sub-frame, but the startpoint of the UE-specific search space may vary per sub-frame dependingon the UE identifier (e.g., C-RNTI), CCE aggregation level and/or slotnumber in a radio frame. In case the start point of the UE-specificsearch space is positioned in the common search space, the UE-specificsearch space and the common search space may overlap.

An aggregation of PDCCH candidates monitored by the UE may be defined onthe basis of a search space. In an aggregation level 1, 2, 4 or 8,search space S_(k) ^((L)) is defined as a set of PDCCH candidates. TheCCE corresponding to PDCCH candidate m in search space S_(k) ^((L)) isgiven as follows:

L{(Y _(k) +m′)mod └N _(CCE,k) /L┘}+i  Equation 1

Here, i=0, . . . L−1, and in case the search space is the common searchspace, m′=m. In case the search space is a specific search space, and acarrier indicator field (CIF) is configured to the UE,m′=m+M^((L))·n_(CI), n_(CI) is a value of the configured CIF. If the CIFis not configured to the UE, m′=m. Here, it is m=0, . . . , M^((L))−1and M^((L)) is the number of the PDCCH candidates for monitoring thegiven search space.

In the common search space, Yk is set as 0 for two aggregation levels,L=4 and L=8. In the UE-specific search space of aggregation level L,variable Yk is defined as follows:

Y _(k)=(A·Y _(k-1))mod D  Equation 2

Here, Y⁻¹=n_(RNTI)≠0, A=39827, D=65537, k=└n_(s)/2┘, and n_(s) is a slotnumber in a radio frame.

When a wireless device monitors the PDCCH based on the C-RNTI, a DCIformat, and a search space are determined according to a PDSCHtransmission mode. Table 12 below shows an example of monitoring PDCCHin which the C-RNTI is configured.

TABLE 3 Trans- mission DCI Search Transmission mode of PDSCH mode formatSpace corresponding to PDCCH Mode 1 DCI Common Single-antenna port, port0 format 1A and UE specific DCI UE Single-antenna port, port 0 format 1specific Mode 2 DCI Common Transmit diversity format 1A and UE specificDCI UE Transmit diversity format 1 specific Mode 3 DCI Common Transmitdiversity format 1A and UE specific DCI UE Cyclic Delay Diversity(CDD)format 2A specific or Transmit diversity Mode 4 DCI Common Transmitdiversity format 1A and UE specific DCI UE Closed-loop spatial format 2specific multiplexing Mode 5 DCI Common Transmit diversity format 1A andUE specific DCI UE Multi-user Multiple Input format 1D specific MultipleOutput(MU-MIMO) Mode 6 DCI Common Transmit diversity format 1A and UEspecific DCI UE Closed-loop spatial format 1B specific multiplexing Mode7 DCI Common If the number of PBCH transmit format 1A and UE ports isone, single-antenna specific port, port 0 is used, otherwise Transmitdiversity DCI UE Single-antenna port, port 5 format 1 specific Mode 8DCI Common If the number of PBCH transmit format 1A and UE ports is one,single-antenna specific port, port 0 is used, otherwise Transmitdiversity DCI UE Dual layer transmit, port 7 or format 2B specific 8 orsingle-antenna port, port 7 or 8

Uses of DCI formats can be classified as shown in the following table.

TABLE 4 DCI format Description DCI used for the scheduling of PUSCHformat 0 DCI used for the scheduling of PDSCH codeword format 1 DCI usedfor the compact scheduling of one PDSCH format 1A codeword and randomaccess procedure DCI used for the compact scheduling of one PDSCH format1B codeword with precoding information DCI used for the compactscheduling of one PDSCH format 1C codeword DCI used for the compactscheduling of one PDSCH format 1D codeword with precoding and poweroffset information DCI used for the scheduling PDSCH to UEs configuredformat 2 in closed-loop spatial multiplexing mode DCI used for thescheduling PDSCH to UEs configured format 2A in open-loop spatialmultiplexing mode DCI used for the transmission of TPC commands forformat 3 PUCCH and PUSCH with 2-bit power adjustments DCI used for thetransmission of TPC commands for format 3A PUCCH and PUSCH with singlebit power adjustment

DCI formats and search spaces to be used may be differently determineddepending on RNTI masked to CRC which has been used for generating DCI.Table 14 below represents DCI formats and search spaces of a controlchannel in case that SI-RNTI, P-RNTI or RA-RNTI is masked to the CRC ofthe DCI.

TABLE 5 DCI Search Transmission mode of PDSCH format space correspondingto PDCCH DCI Common If the number of PBCH transmit ports is format 1Cone, single-antenna port, port 0 is used, otherwise Transmit diversityDCI Common If the number of PBCH transmit ports is format 1A one,single-antenna port, port 0 is used, otherwise Transmit diversity

Table 6 below shows DCI formats and search spaces of a control channelin case that SPS-C-RNT is masked to the CRC of the DCI

TABLE 6 Trans- Transmission mode of mission DCI Search PDSCHcorresponding mode format space to PDCCH Mode 1 DCI Common Singleantenna port, format 1A and UE port 0 specific DCI UE Single antennaport, format 1 specific port 0 Mode 2 DCI Common Transmit diversityformat 1A and UE specific DCI UE Transmit diversity format 1 specificMode 3 DCI Common Transmit diversity format 1A and UE specific DCI UETransmit diversity format 2A specific Mode 4 DCI Common Transmitdiversity format 1A and UE specific DCI UE Transmit diversity format 2specific Mode 5 DCI Common Transmit diversity format 1A and UE specificMode 6 DCI Common Transmit diversity format 1A and UE specific Mode 7DCI Common Single antenna port 5 format 1A and UE specific DCI UE Singleantenna port 5 format 1 specific Mode 8 DCI Common Single antenna port 7format 1A and UE specific DCI UE Single antenna port 7 or 8 format 2Bspecific Mode 9 DCI Common Single antenna port 7 format 1A and UEspecific DCI UE Single antenna port 7 or 8 format 2C specific Mode 10DCI Common Single antenna port 7 format 1A and UE specific DCI UE Singleantenna port 7 or 8 format 2D specific

Table 7 below shows search spaces and DCI formats used in case thattemporary C-RNTI is masked to the CRC of the DCI.

TABLE 7 DCI Search Transmission mode of PDSCH corresponding format spaceto PDCCH DCI Common If the number of PBCH transmit ports is format 1Aand UE one, single-antenna port, port 0 is specific used, otherwiseTransmit diversity DCI Common If the number of PBCH transmit ports isformat 1 and UE one, single-antenna port, port 0 is specific used,otherwise Transmit diversity

FIG. 8 shows an example of a downlink subframe in which a referencesignal and a control channel in a wireless communication system to whichthe present invention is applied.

Refer ti FIG. 8, a downlink subframe may be classified into a controlregion and a data region. For example, in the downlink subframe, thecontrol region (or a PDCCH region) includes front three OFDM symbols andthe data region in which a PDSCH is transmitted includes remaining OFDMsymbols.

In the control region, a PCFICH, a PHICH and/or the PDCCH aretransmitted. The physical HARQ ACK/NACK indicator channel (PHICH) maytransmit a hybrid automatic retransmission request (HARQ) information asa response to a uplink transmission. The physical control formatindicator channel (PCFICH) may transmit the information of the number ofOFDM symbols allocated to the PDCCH. For example, a control formatindicator (CFI) of the PCFICH may indicate three OFDM symbols. Theregion excluding the resource through which the PCFICH and/or the PHICHis transmitted is the PDCCH region that a wireless device monitors thePDCCH.

In the subframe, various reference signals may be transmitted as well. Acell-specific reference signal reference signal (CRS) is a referencesignal that all wireless devices in a cell may receive, and may betransmitted over the whole downlink frequency band. In FIG. 8, R0denotes an RE (resource element) where a CRS for a first antenna port istransmitted, R1 which is an RE where a CRS for a second antenna port istransmitted, R2 which is an RE where a CRS for a third antenna port istransmitted, and R3 which is an RE where a CRS for a fourth antenna portis transmitted. The RS sequence r_(l,n) _(s) (m) for CRS is defined asfollows.

$\begin{matrix}{{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2m} + 1} \right)}}} \right)}}} & {\langle{{Equation}\mspace{14mu} 3}\rangle}\end{matrix}$

Herein, m=0, 1, . . . , 2N_(RB)N_(max,DL) is the maximum number of RBs,ns is a slot number in a radio frame, and 1 is an OFDM symbol index in aslot.

A pseudo-random sequence, c(i), is defined by a gold sequence whoselength is 31, as follows.

c(n)=(x ₁(n+N _(C))+x ₂(n+N _(C)))mod 2

x ₁(n+31)=(x ₁(n+3)+x ₁(n))mod 2

x ₂(n+31)=(x ₂(n+3)+x ₂(n+2)+x ₂(n+1)+x ₂(n))mod 2  <Equation 4>

Herein, Nc=1600, and the first m-sequence is initialized as x1(0)=1,x1(n)=0, m=1, 2, . . . , 30. The second m-sequence is initialized asc_(init)=2¹⁰·(7·(n_(s)+1)+l+1)·(2·N_(ID) ^(cell)+1)+2·N_(ID)^(cell)+N_(CP) at the beginning of each OFDM symbol. is a physical cellidentity (PCI) of the cell, and N_(CP)=1 in case of the normal CP, andN_(CP)=0 in case of the extended CP.

Also, a UE-specific reference signal (URS) may be transmitted in asubframe. Although the CRS is transmitted in the entire region of asubframe, the URS is transmitted in the data region of the sub-frame,and is a reference signal used for demodulating the PDSCH. In FIG. 8, R5denotes an RE where the URS is transmitted. A DM-RS is a referencesignal used for demodulating the EPDCCH data.

The URS may be transmitted in an RB in which the corresponding PDSCHdata is mapped. Although in FIG. 8, R5 is denoted outside the area inwhich the PDSCH is transmitted, this is merely to indicate the positionof the RE to which the URS is mapped.

The URS is may be a reference signal which is demodulated only by aspecific wireless device. The RS sequence r_(l,n) _(s) (m) for the URSis the same as Equation 3. At this time, m=0, 1, . . . , 12N_(RB)^(PDSCH)−1 and N_(RB) ^(PDSCH) is the number of RBs which is used forthe corresponding PDSCH transmission. In case that the URS istransmitted through a single antenna, the pseudo-random sequencegenerator is initialized as c_(init)=(└n_(s)/2┘+1)·(2N_(ID)^(cell)+1)·2¹⁶+n_(RNTI) at the start of each subframe. n_(RNTI) is anidentifier of a wireless device.

The above-described initializing method is associated with the casewhere the URS is transmitted through a single antenna. When the URS istransmitted through a multi-antenna, the pseudo-random sequencegenerator is initialized as c_(init)=(└n_(s)/2┘+1)·(2N_(ID)^(cell)+1)·2¹⁶+n_(SCID) at the start of each sub-frame. n_(SCID) is aparameter that is acquired from a DL grant (for example, DCI format 2Bor 2C) related with PDSCH transmission.

The URS supports multiple input multiple output (MIMO) transmission.Depending on an antenna port or layer, the RS sequence for the URS maybe spread to the spread sequence as follows.

TABLE 8 Layer [w(0), w(1), w(2), w(3)] 1 [+1 +1 +1 +1] 2 [+1 −1 +1 −1] 3[+1 +1 +1 +1] 4 [+1 −1 +1 −1] 5 [+1 +1 −1 −1] 6 [−1 −1 +1 +1] 7 [+1 −1−1 +1] 8 [−1 +1 +1 −1]

A layer may be defined as an information path inputted to a pre coder. Arank is the number of non-zero eigenvalue in the MIMO channel matrix,and is the same as the number of layer or space stream. The layer maycorrespond to an antenna port that distinguishes the URS and/or a spreadsequence which is applied to the URS.

Meanwhile, the PDCCH is monitored in a restricted region such as acontrol region in a subframe, and the CRS transmitted from whole bandsis used for demodulating the PDCCH. As the sort of control data becomesdiverse and an amount of the control data is increased, a flexibility ofscheduling becomes deteriorated with the existing PDCCH only. Also, inorder to decrease overhead owing to the CRS transmission, an enhancedPDCCH (EPDCCH) is introduced.

FIG. 9 is a view illustrating an exemplary subframe with EPDCCH.

The subframe may include 0 or 1 PDCCH region 910 and 0 or more EPDCCHregions 920 and 930.

The EPDCCH regions 920 and 930 are regions where a UE monitors EPDCCH.The PDCCH region 910 is located in preceding 3 or up to 4 OFDM symbolsof a subframe, and The EPDCCH regions 920 and 930 may be flexiblyscheduled in the OFDM symbols, following the PDCCH region 910.

One or more EPDCCH regions 920 and 930 may be assigned to the UE. The UEmay monitor EPDCCH data in the EPDCCH regions 920 and 930 assigned tothe UE.

A base station may notify the UE of information on a subframe formonitoring the EPDCCH and/or the number/position/size of the EPDCCHregions 920 and 930 through a radio resource control (RRC) message, andthe like.

In the PDCCH region 910, the PDCCH can be demodulated based on CRS. Inthe EPDCCH regions 920 and 930, DM-RS may be defined rather than CRS fordemodulation. The DM-RS may be transmitted in the corresponding EPDCCHregions 920 and 930.

A RS sequence for the DM-RS is expressed in Equation 3. Here, m=0, 1, .. . , 12N_(RB) ^(max,DL)−1 and N_(RB) ^(max,DL) is the maximum number ofRBs. A pseudo-random sequence generator can be initialized asc_(init)=(└n_(s)/2┘+1)·(2n_(ID,i) ^(EPDCCH)+1)·2¹⁶+n_(SCID) ^(EPDCCH) atthe start of each subframe. ns is the number of a slot in a radio frame,n_(ID,i) ^(EPDCCH) is a cell index related to the corresponding EPDCCHregion, and n_(SCID) ^(EPDCCH) is a parameter given from higher-layersignaling.

Each of the EPDCCH regions 920 and 930 may be used in scheduling fordifferent cells. For example, EPDCCH within the EPCCH region 920 candeliver information on scheduling for a primary cell, and EPDCCH withinthe EPCCH region 930 can send information on scheduling for a secondarycell.

When the EPDCCH is transmitted via multiple antenna in the EPDCCHregions 920 and 930, the same precoding as that of the EPDCCH may beapplied to DM-RS in EPDCCH regions 920 and 930.

Considering that the PDCCH uses CCE as a transmission resource unit, atransmission resource unit for the EPDCCH is referred to as EnhancedControl Channel Element (ECCE), An aggregation level may be defined as aresource unit for monitoring the EPDCCH. For example, assuming that 1ECCE is a minimum resource for the EPDCCH, an aggregation level may beL={1, 2, 4, 8, 16}. A search space may be defined even in the EPDCCHregion. The UE can monitor EPDCCH candidates on the basis of theaggregation level.

FIG. 10 is a conceptual diagram showing a carrier aggregation.

FIG. 10(A) shows a single component carrier (CC). A single CC maycorrespond to an uplink frequency band 1000 and a downlink frequencyband 1020 of 20 MHz. FIG. 10(B) shows multiple CCs. For example, themultiple CC may correspond to an uplink frequency band 1040 and adownlink frequency band 1060 of 60 MHz in which the uplink frequencyband and the downlink frequency band of 20 MHz are aggregated.

A BS may transmit data to a wireless device through the plurality ofdownlink CCs by performing a carrier aggregation. The BS may perform adownlink transmission using N downlink CCs. In this time, if a wirelessdevice may receive downlink data through only M (M is a natural numbersmaller than or equal to N) downlink CCs, the wireless device mayreceive the downlink data which are transmitted through only the Mdownlink CCs from the BS.

Additionally, a BS may set a frequency bandwidth that corresponds to L(L is a natural number smaller than or equal to M and N) downlink CCs asa main CC and operate the frequency bandwidth. The wireless device maypreferentially monitor and receive the data that the BS transmitsthrough a main CC. In case of performing the carrier aggregation, a CCmay be distinguished according to a cell.

In case of performing the carrier aggregation using the CC of a primarycell (P-cell) and the CC of a secondary cell (S-cell), a carrier thatcorresponds to the CC of a P-cell among the component carriers used indownlink and uplink is called a primary cell component carrier (PCC) anda carrier that corresponds to the CC of S-cell is called a second cellcomponent carrier (SCC).

FIG. 11 is a conceptual diagram showing the P-cell and the S-cell.

Referring to FIG. 11, a BS may perform a carrier aggregation based onthe PCC of a P-cell 1100 and the SCC of one or more S-cell 1120. In casethat two or more cells exist, the BS may determine one cell to be theP-cell 1100 and other cells to be S-cell 1020. The BS may aggregate theCCs of the determined P-cell 1100 and the S-cell 1120, and transmit datato a wireless device using an aggregated frequency bandwidth. Thewireless device may also transmit data to the BS using the aggregatedfrequency bandwidth. As an exemplary case among the scenarios in whichthe P-cell 1100 and the S-cell 1110 are deployed, the P-cell and theS-cell 1120 shown in FIG. 11 shows the case that a transmission range ofthe data transmitted based on the PCC of the P-cell 1100 is greater thana transmission range of the data transmitted based on the SCC of theS-cell 1120.

The wireless device may perform the radio resource control (RRC)connection through the PCC of the P-cell 1100. Furthermore, the wirelessdevice may attempt to perform a random access to the BS through aphysical random access channel (PRACH) based on a signal signaledthrough the PCC. That is, the wireless device may perform an initialconnection establishment process or a connection re-establishmentprocess to the BS through the PCC in the carrier aggregationenvironment.

The SCC of the S-cell 1120 may be used for providing additional radioresources. In order to perform the carrier aggregation that adds the SCCto the PCC, the wireless device should perform a neighbor cellmeasurement that the wireless device acquires the information ofneighboring cells. Based on the neighbor cell measurement performed bythe wireless device, the BS may determine whether to aggregate the SCCinto the PCC. For example, a legacy subframe is transmitted through aPCC in a Pcell, and a new subframe (to be described hereinafter)effectively reducing a control channel and reference signals transmittedin the legacy subframe, as well as using the legacy subframe, may betransmitted in an Scell. In a new LTE-A release, a subframe of a newformat may be defined to be used. Hereinafter, for the purposes ofdescription, a newly defined subframe different from an existingsubframe may be defined as a new carrier (NC) subframe.

That is, in an existing LTE release 8/9/10 system, control channels suchas CRS, PSS/SSS, PDCCH, or PBCH, a reference signal, and asynchronization signal may be transmitted in a downlink carrier. Asubframe in which such control channels, a reference signal, and asynchronization signal are defined may be called a legacy subframe. In asystem after the LTE release 8/9/10 system, a portion of channels orsignals transmitted in the existing legacy subframe may not betransmitted to improve interference among a plurality of cells andenhance carrier expandability. A subframe having such characteristicsmay be defined as an extension carrier subframe or an NC subframe andused. For example, an NC subframe may not include a control channeland/or reference signal information such as PDCCH data and CRS. Forexample, when a PDCCH does not exist in an NC subframe, controlinformation may be transmitted through an EPDCCH. A PDSCH of an NCsubframe may be allocated on the basis of an EPDCCH included in an NCsubframe. Thus, a legacy subframe may be a subframe used to bedistinguished from a subframe format defined before the 3GPP LTE-Arelease 11 or a subframe newly defined in the 3GPP LTE-A release 12.

A base station (BS) may transmit PDCCH data to a terminal through a PCC.The PDCCH data may include allocation information regarding PDSCH datatransmitted through a downlink PCC band and SCC band and informationapproving data transmission through uplink. A Pcell 1100 and an Scell1120 may perform CA through a configuration and activation operation andtransmit and receive data through a corresponding frequency band. Here,the Pcell is a carrier activated all the same, and the Scell may operateaccording to an activation/deactivation instruction from the BS, and theactivation/deactivation may be instructed in the form of a MAC message.

FIG. 12 is a conceptual diagram showing a method of transmitting data toa wireless device based on a coordinated multi points (CoMP) in aplurality of transmission points.

Referring FIG. 12, traffic data and control data may be transmitted to awireless device based on a CoMP at a plurality of transmission points.The plurality of transmission points may generate data which aretransmitted to a wireless device within a cell based on a cell ID whichis identical or different. The plurality of transmission points may becalled a plurality of serving cells or cells in other terminology, andthe CoMP may transmit or receive data based on serving cells which aredifferent from each other.

A method is shown that a first transmission point 1210 and a secondtransmission point 1220 transmit data to a wireless device using a jointtransmission (JT) method of the CoMP. In case that the plurality oftransmission points 1210 and 1220 transmit data to the wireless device1200 using the JT method, the same data may be transmitted to thewireless device 1200 from different transmission points 1210 and 1220.The wireless device 1200 may receive and demodulate the data transmittedfrom different transmission points 1210 and 1220.

A third transmission point 1230 and a fourth transmission point 1240 maytransmit data to a wireless device 1250 using a dynamic point selection(DPS) method of the CoMP. In the DPS method, the wireless device mayreceive data by dynamically selecting a transmission point having abetter channel from the transmission points 1230 and 1240 different fromeach other. For example, when transmitting EPDCCH data to the wirelessdevice 1250 from the third transmission point 1230 on a first time,EPDCCH data may be transmitted to the wireless device 1250 from thefourth transmission point 1240 on a second time.

FIG. 13 shows a transmission of a synchronization signal and PBCH datain a legacy subframe when Frequency Division Duplexing (FDD) is used inaccording to a duplexing method.

A physical broadcast channel (PBCH) 1300 is transmitted in former fourOFDM symbols in a second slot 1350-2 in the first subframe (i.e.,subframe 1350 having an index is 0) of a radio frame. The PBCH 1300carries system information essential for a wireless device tocommunicate with a BS, and system information transmitted through thePBCH 1300 is called a master information block (MIB). In contrast,system information transmitted on a PDSCH that is indicated by a PDCCHis called a system information block (SIB).

Seventh OFDM symbols (i.e., OFDM symbol having an index 6), from amongOFDM symbols allocated to the first slots 1350-1 and 1370-1 of the firstsubframe (i.e., subframe 1350 having an index 0) and a seventh subframe(i.e., subframe 1370 having an index 5), may include respective primarysynchronization signals (PSSs) 1320 and 1325. The PSSs 1320 and 1325 maybe used for acquiring OFDM symbol synchronization or slotsynchronization. Furthermore, the information of a physical cell ID maybe acquired through the PSSs 1320 and 1325. A primary synchronizationcode (PSC) is a sequence which is used for generating the PSSs 1320 and1325. The PSS may be generated by defining a plurality of PSCs in 3GPPLTE. A BS may generate the PSSs 1320 and 1325 using one of 3 PSCs basedon a cell ID. A wireless device may acquire the information of the cellID based on the PSC by receiving the PSSs 1320 and 1325.

Seventh OFDM symbols (i.e., OFDM symbol having an index 6), from amongOFDM symbols allocated to the first slots 1350-1 and 1370-1 of the firstsubframe (i.e., subframe 1350 having an index 0) and a seventh subframe(i.e., subframe 1370 having an index 5), may include secondarysynchronization signals (SSSs) 1310 and 1315.

The first SSS 1310 may be transmitted through sixth OFDM symbol in thefirst slot 1350-1 of the first subframe 1350 and the second SSS 1325 maybe transmitted through sixth OFDM symbol in the first slot 1370-1 of thesixth subframe 1370. The SSSs 1310 and 1315 may be used for obtain framesynchronization. The SSSs 1310 and 1315 are used for acquiringinformation of a cell ID together with the PSSs 1310 and 1315.

The first SSS 1310 and the second SSS 1315 may be generated usingdifferent secondary synchronization codes (SSCs). When each of the firstSSS 1310 and the second SSS 1315 includes 31 subcarriers, each of thetwo SSC sequences whose length is 31 is used for the first SSS 1310 andthe second SSS 1315.

From a viewpoint of a frequency domain, the PBCH 1300, the PSSs 1310 and1320, and the SSSs 1315 and 1325 are transmitted within a frequencybandwidth that corresponds to 6 RBs on the basis of a center frequencyof the subframe. Meanwhile, a case in which a legacy subframe and an NCsubframe are transmitted together in a plurality of transmission points(TP) may be assumed. In this case, information regarding allocation of aPDSCH transmitted through the NC subframe may be included even in aPDCCH included in a legacy subframe. In the NC subframe, downlinkcontrol information such as a DCI may be transmitted through an EPDDCH.In the NC subframe, a CRS is not transmitted, and thus, the DCI may bedemodulated on the basis of a reference signal such as a DM-RS. The NCsubframe may be NC subframe even when configuration of the NC subframeand a legacy subframe are configured in a time division multiplexing(TDM) manner in a single subframe. For example, even when one slot isgenerated as configuration of a channel and a signal of an NC subframeand the other one slot is generated as configuration of a channel and asignal of a legacy subframe, the corresponding subframe may beconsidered an NC subframe. Also, the NC subframe and the legacy subframemay be divided on the basis of a time within a single frame in a TDMmanner and transmitted. For example, a frame transmitted in a singlecell may include both an NC subframe and a legacy subframe, which may beconsidered an NC frame.

On the assumption of a Pcell transmitting data on the basis of a legacysubframe and an Scell transmitting data using an NC subframe, data maybe transmitted to a terminal on the basis of the Pcell and the Scell.That is, the NC subframe may be a subframe transmitting in an SCC, afrequency band allocated to the Scell. When data is transmitted to aterminal on the basis of the Pcell and the Scell, a BS may inform theScell about a position of an OFDM symbol in which a PDSCH starts in alegacy subframe through higher layer signaling. A parameter informingabout the position of the OFDM symbol in which PDSCH starts in thelegacy subframe may have values from 1 to 4.

An NC frame including NC subframes may include ten NC subframes. The NCframe may transmit a reference signal performing time/frequency trackingonly in a particular subframe, rather than in every subframe included inthe frame. The reference signal performing time/frequency tracking,included in the NC subframe and transmitted may be a tracking referencesignal (TRS). Instead of the TRS, a term of enhanced synchronizationsignal (eSS) or a reduced CRS may be used to express the referencesignal performing time/frequency tracking, included in the NC subframeand transmitted. The TRS may be transmitted in a particular subframe(for example, subframe 0 to subframe 5) of the single NC frame. The TRSmay be a reference signal defined to be transmitted in a particular RERof a particular RB of the NC subframe. Alternatively, an RS having a newform or an added DM-RS for a synchronization signal (discovery channels)may be used in the NC subframe.

In the NC subframe, PDSCH data may not be mapped to a TRS-configured REand transmitted. That is, in the NC subframe, data rate matching may beperformed on PDSCH data in consideration of a TRS-configured RE. AnotherNC subframe may be a subframe in the form in which the TRS-configured REis punctured.

An antenna port for transmitting the TRS may be defined as an antennaport x. When the BS transmits the TRS on the basis of the antenna portx, the BS may not map data of PDSCH or EPDCCH in an RE corresponding tothe antenna port x transmitting the TRS.

An initial value of a pseudo-random sequence used to generate the TRSmay be determined on the basis ofc_(init)=2¹⁰·(7·(n_(s)+₁)+l+1)·(2·N_(ID) ^(cell)+1)+2·N_(ID)^(cell)+N_(CP). Here, n_(s) denotes a slot number, l denotes the numberof an OFDM symbol, N_(ID) ^(cell) denotes a cell identifier, and N_(CP)denotes a length of a CP. N_(CP) may have different values according totypes of the CP.

As a parameter for reducing an influence of inter-cell interference,v-shift may be used. V-shift may be used as a parameter for adjusting aposition of the RE to which the TRS is mapped. For example, v-shift maybe determined on the basis of v_(shift)=N_(ID) ^(cell) mod 6. V-shiftmay be a fixed value such as 0.

FIG. 14 is a concept view illustrating transmission of a CSI-RS and aCSI feedback measured by a terminal to which the present invention isapplied.

Referring to FIG. 14, the terminal 1410 may feed back to the basestation 1400 channel information produced based on a CSI-RS transmittedfrom the base station 1400 using parameters such as an RI (rank index),a PMI (precoding matrix index), or a CQI (channel quality indicator).The parameters indicating channel information, such as an RI, a PMI, ora CQI, may be denoted CSI (channel state information) feedbackinformation. Each type of CSI feedback information may play a role asfollows:

(1) RI (rank index) may contain information on a transmission rank. Inother words, information on the number of layers used for downlinktransmission may be provided to the base station based on the RI.

(2) PMI (precoding matrix index) may contain information on a precodingmatrix used for downlink transmission.

(3) CQI (channel-quality indication) may contain information on an MCS(modulation and coding scheme).

The terminal 1410 may report information on the downlink channel stateby transmitting the RI, PMI, CQI or other information indicating thechannel state, as the feedback information for the CSI-RS transmittedfrom the base station 1400.

The CRS is also a reference signal that may be used for the terminal toobtain downlink channel state information. Accordingly, the CRS mayoverlap, in role, the CSI-RS. The CSI-RS may be used to supplement theCRS, an existing reference signal. As the number of transmit antennasincreases, the CSI-RS may be used to determine better the channel stateinformation than the existing reference signal, CRS. The existing CRSdensity was set high in order to enable channel measurement in the veryquickly varying channel environment. Accordingly, the CRS operates as ahigh overhead.

In contrast, the CSI-RS is a reference signal used only to obtain CSI,and thus, the CSI-RS has low time-frequency density. Accordingly, theCSI-RS has a lower overhead than the CRS. Therefore, as a new type ofreference signal, rather than extensions to the existing referencesignal, CRS, the CSI-RS having low time-frequency density and lowoverhead may be defined and used.

One cell or base station may include one, two, four, or eight CSI-RSsfor each resource block pair, and may transmit the same to the terminalA CSI-RS configuration is a deployment of CSI-RSs in a resource grind,and there may be different CSI-RS configurations depending on the numberof CSI-RSs used in one cell.

FIG. 15 is a concept view illustrating a downlink transport channelprocessing method to which the present invention is applied. FIG. 15illustrates an operation in which a transport block is transmitted via atransport channel to a physical layer.

Referring to FIG. 15, an LTE physical layer interfaces with its higherlayer, an MAC layer, by way of a transport channel. In the case ofsingle antenna transmission, there is a dynamically-sized transportblock per TTI (transmission time interval). For example, in the case ofmulti-antenna transmission, there may be multiple (e.g., two)dynamically-sized transport blocks per TTI.

FIG. 15 illustrates a processing procedure for DL-SCH transmission inconducting an LTE downlink transmission process. The second processingprocedure corresponding to the second transport block is provided onlyin the case of downlink spatial multiplexing. In the case of spatialmultiplexing, two different-size transport blocks may be typicallycombined with each other through antenna mapping. The LTE downlinktransport channel processing method illustrated in FIG. 15 is nowdescribed.

(1) Insertion of CRC Per Transport Block

At the first step of the transport channel processing, a 24-bit CRC maybe computed and the same may be added to each transport block. Errors inthe decoded transport blocks may be detected at the reception endthrough the CRC. For example, a downlink HARQ protocol may be used toinform the detected errors and to request re-transmission.

(2) Segmentation of Code Block and Insertion of CRC Per Code Block

The interleaver in the LTE turbo code may be restricted as per size, andthe same may be defined only for a limited size of code blocks having aspecific bit in the maximum block size. In case the size of theCRC-added transport block is more than the maximum code block size, codeblock segmentation may be conducted before turbo coding is conducted.The code block segmentation refers to dividing the transport block intosmaller code blocks that fit the code block size defined in the turbocode.

(3) Turbo Coding

In LTE systems, the WCDMA/HSPA turbo encoder internal interleaver hasbeen replaced with QPP (quadrature permutation polynomial)-basedinterleaving. Contrary to the WCDMA/HSPA turbo code interleaver, theQPP-based interleaver is a maximally contention-free interleaver, andthus, the QPP-based interleaver may enable simple parallelization of adecoding process without collision even when different parallelprocesses approach the interleaver memory.

(4) Rate Matching and Physical Layer HARQ Function.

Rate matching and physical layer HARQ are for correct selection of bitsto be transmitted within a given TTI from the blocks of the code bitstransferred from the channel encoder. The outputs from the turbo encoder(systematic bits, first parity bits, and second parity bits) each may befirst subjected to interleaving. The interleaved bits may enter thecircular buffer. The bit selection block extracts as many consecutivebits as the allocated resources from the circular buffer.

(5) Per-Bit Scrambling

LTE downlink scrambling refers to multiplying the blocks of code bitsthat have undergone the rate matching and HARQ by a per-bit scramblingsequence. In LTE systems, downlink scrambling may apply to code bits ofeach transport channel.

(6) Data Modulation

Downlink data modulation denotes a process of transforming scrambledbits into corresponding complex modulated symbols. The LTE downlinksupports the following modulation schemes: QPSK, 16QAM, and 64 QAM.According to an embodiment of the disclosure, an example in which 256QAM is also supported as an additional modulation scheme is described.In the modulation schemes, QPSK, 16QAM, and 64QAM respectively maycorrespond to two bits per symbol, four bits per symbol, and six bitsper symbol. Different modulation schemes may be put in use depending ontransport channels.

(7) Antenna Mapping

Typically, antenna mapping simultaneously processes modulation symbolscorresponding to two transport blocks and maps the processed results todifferent antenna ports.

(8) Resource Block Mapping

Resource block mapping maps symbols to be transmitted through respectiveantenna ports to resource elements of resource blocks allocated totransport blocks transmitted to the terminal by an MAC scheduler.

Some resource elements in the resource blocks may be pre-occupied byother antenna port or control region, and such resource elements cannotbe put in use.

In order to transmit a data block size to the terminal, the BS may use adownlink control channel (e.g., PDCCH and EPDCCH). The BS may transmitinformation regarding a data block size transmitted through the PDSCH onthe basis of MCS and resource allocation information as modulation andcoding rate-related information. The MCS field may transmit MCSinformation on the basis of 5 bits, for example, to the terminal. As forresource allocation, 1RB to 110RB may be allocated. In a case in whichall of 5 bits of the MCS field are used to transmit the MCS informationwithout using MIMO, 32 pieces of MCS information may be transmitted onthe basis of 5 bits. In this case, a data block size corresponding to32×110 may be signaled. However, three pieces of MCS information, amongthe 32 pieces of MCS information, is used to indicate a change in amodulation scheme when performing retransmission, and thus, inactuality, a data block size corresponding to 29×110 may be signaled. Adata block may refer to a transmission block.

As a modulation scheme supported in the LTE system to which the presentinvention is applied, QPSK, 16QAM, or 64QAM may be used. At a switchingpoint where a modulation scheme is changed, in a case in which the sameresource has been allocated, the same data block size may be indicated.This is to effectively perform an operation in various channelenvironments. In order to indicate an actual data block size, IMCS,MCS-related information transmitted through a downlink control channel,may be mapped to an ITBS, another variable for indicating a data blocksize. Table 9 below illustrates a relationship between I_(MCS) andI_(TBS).

TABLE 9 MCS Index Modulation Order TBS Index I_(MCS) Q_(m) I_(TBS) 0 2 01 2 1 2 2 2 3 2 3 4 2 4 5 2 5 6 2 6 7 2 7 8 2 8 9 2 9 10 4 9 11 4 10 124 11 13 4 12 14 4 13 15 4 14 16 4 15 17 6 15 18 6 16 19 6 17 20 6 18 216 19 22 6 20 23 6 21 24 6 22 25 6 23 26 6 24 27 6 25 28 6 26 29 2reserved 30 4 31 6

A transmission block size transmitted to downlink may be determined by acombination of an MCS field transmitted in a downlink control channeland resource allocation. Table 10 and Table 11 below illustratetransmission block sizes in the I_(MCS) to I_(TBS) relationship of Table8 in case of 10 RB resource allocation in 1 RB and in case of 110 RBresource allocation in 101 RB.

TABLE 10 N_(PRB) I_(TBS) 1 2 3 4 5 6 7 8 9 10 0 16 32 56 88 120 152 176208 224 256 1 24 56 88 144 176 208 224 256 328 344 2 32 72 144 176 208256 296 328 376 424 3 40 104 176 208 256 328 392 440 504 568 4 56 120208 256 328 408 488 552 632 696 5 72 144 224 328 424 504 600 680 776 8726 328 176 256 392 504 600 712 808 936 1032 7 104 224 328 472 584 712 840968 1096 1224 8 120 256 392 536 680 808 968 1096 1256 1384 9 136 296 456616 776 936 1096 1256 1416 1544 10 144 328 504 680 872 1032 1224 13841544 1736 11 176 376 584 776 1000 1192 1384 1608 1800 2024 12 208 440680 904 1128 1352 1608 1800 2024 2280 13 224 488 744 1000 1256 1544 18002024 2280 2536 14 256 552 840 1128 1416 1736 1992 2280 2600 2856 15 280600 904 1224 1544 1800 2152 2472 2728 3112 16 328 632 968 1288 1608 19282280 2600 2984 3240 17 336 696 1064 1416 1800 2152 2536 2856 3240 362418 376 776 1160 1544 1992 2344 2792 3112 3624 4008 19 408 840 1288 17362152 2600 2984 3496 3880 4264 20 440 904 1384 1864 2344 2792 3240 37524136 4584 21 488 1000 1480 1992 2472 2984 3496 4008 4584 4968 22 5201064 1608 2152 2664 3240 3752 4264 4776 5352 23 552 1128 1736 2280 28563496 4008 4584 5160 5736 24 584 1192 1800 2408 2984 3624 4264 4968 55445992 25 616 1256 1864 2536 3112 3752 4392 5160 5736 6200 26 712 14802216 2984 3752 4392 5160 5992 6712 7480

TABLE 11 N_(PRB) I_(TBS) 101 102 103 104 105 106 107 108 109 110 0 27922856 2856 2856 2984 2984 2984 2984 2984 3112 1 3752 3752 3752 3752 38803880 3880 4008 4008 4008 2 4584 4584 4584 4584 4776 4776 4776 4776 49684968 3 5992 5992 5992 5992 6200 6200 6200 6200 6456 6456 4 7224 72247480 7480 7480 7480 7736 7736 7736 7992 5 8760 9144 9144 9144 9144 95289528 9528 9528 9528 6 10680 10680 10680 10680 11064 11064 11064 1144811448 11448 7 12216 12576 12576 12576 12960 12960 12960 12960 1353613536 8 14112 14112 14688 14688 14688 14688 15264 15264 15264 15264 915840 16416 16416 16416 16416 16992 16992 16992 16992 17568 10 1756818336 18336 18336 18336 18336 19080 19080 19080 19080 11 20616 2061620616 21384 21384 21384 21384 22152 22152 22152 12 22920 23688 2368823688 23688 24496 24496 24496 24496 25456 13 26416 26416 26416 2641627376 27376 27376 27376 28336 28336 14 29296 29296 29296 29296 3057630576 30576 30576 31704 31704 15 30576 31704 31704 31704 31704 3285632856 32856 34008 34008 16 32856 32856 34008 34008 34008 34008 3516035160 35160 35160 17 36696 36696 36696 37888 37888 37888 39232 3923239232 39232 18 40576 40576 40576 40576 42368 42368 42368 42368 4381643816 19 43816 43816 43816 45352 45352 45352 46888 46888 46888 46888 2046888 46888 48936 48936 48936 48936 48936 51024 51024 51024 21 5102451024 51024 52752 52752 52752 52752 55056 55056 55056 22 55056 5505655056 57336 57336 57336 57336 59256 59256 59256 23 57336 59256 5925659256 59256 61664 61664 61664 61664 63776 24 61664 61664 63776 6377663776 63776 66592 66592 66592 66592 25 63776 63776 66592 66592 6659266592 68808 68808 68808 71112 26 75376 75376 75376 75376 75376 7537675376 75376 75376 75376

FIG. 16 is a conceptual view illustrating a PRACH used for random accessof a terminal to which the present invention is applied.

Referring to FIG. 16, a PRACH is a preamble transmitted by a terminal toa BS during a random access process. One PRACH may be transmitted in 6RB per subframe in FDD. A plurality of terminals may be transmittedthrough the same PRACH resource 1600 using different preambles. It isdetermined whether to transmit the PRACH in each subframe or in everysubframe according to setting of a transmission period. The resource1600 in which the PRACH may be transmitted may be known on the basis ofPRACH configuration information (PRACH-configuration index). The PRACHconfiguration index may have values from 0 to 63. n^(RA) _(PRBoffset)may be a parameter for indicating a start frequency specified in anetwork.

FIG. 17 is a conceptual view illustrating a PRACH to which the presentinvention is applied.

Referring to FIG. 17, the PRACH may be transmitted with differentlengths of different sequences according to preamble formats. The PRACHmay be generated on the basis of 64 available preamble sequences. Apreamble may include a CP and a preamble sequence. The CP is a guardspace for handling uncertainty of a timing. The preamble sequence may begenerated on the basis of a cyclic-shifted Z-C (Zaddoff-Chu) sequence.

Table 12 below illustrates a random access preamble format.

TABLE 12 Preamble format T_(CP) T_(SEQ) 0  3168 · T_(s) 24576 · T_(s) 121024 · T_(s) 24576 · T_(s) 2  6240 · T_(s) 2 · 24576 · T_(s) 3 21024 ·T_(s) 2 · 24576 · T_(s) 4  448 · T_(s)  4096 · T_(s) (frame structuretype 2 only)

Referring to Table 12, a length of a transmitted preamble may varyaccording to preamble formats.

The preamble format 0 may be a preamble format transmitted in a generalenvironment (e.g., within 15 kilometers of cell radius). A preambleformat 2 and the preamble format 3 may be used in a situation which anSINR is low. In the preamble format 2 and the preamble format 3, asequence may be repeatedly transmitted. A preamble format 4 may be apreamble format used in a TDD mode.

In a next system of LTE-A, a low-priced/low-specification terminaloriented toward data communication such as reading an electric meter,measurement of a water level, utilization of a monitoring camera,reporting of stock of a vending machine, and the like, is considered.Hereinafter, in an embodiment of the present invention, such a terminalmay be defined as a term of a machine type communication (MTC) terminalfor the purposes of description. The MTC terminal has characteristicsthat a transmission data amount is small and data transmission andreception through uplink and downlink occurs intermittently, rather thancontinuously. Thus, The MTC terminal is required to be lowered in priceaccording to the low data transfer rate and reduce battery consumption.Since data traffic of the MTC terminal occurs intermittently, ratherthan continuously, an existing channel measurement method and anexisting channel measurement result reporting method may not beeffective in performing channel estimation.

Also, since data transmission and reception is performed intermittentlyin the MTC terminal, when data transmission and reception is performedin the MTC terminal once, an operation mode of the MTC terminal may becontrolled to be switched to a sleep mode. Thus, it may be moreeffective for the MTC terminal to perform reliable transmission, ratherthan employing a procedure of transmission and reception a plurality oftimes based on an ACKK/NACK process. The MTC device may be installed inan area such as a limited underground, an indoor area, the interior of abuilding, and the like, and thus, coverage thereof may be limited. Thus,in order to ensure reliable transmission, downlink transmission coverageand uplink transmission coverage of the MTC terminal needs to beenhanced.

Hereinafter, an embodiment of the present invention is to enhance uplinktransmission coverage and a downlink transmission coverage of an MTCterminal and provide a scheme of performing reliable transmissionwithout having to perform an HARQ process. In case of using the HARQprocess, a correction may be required, but the present invention may beapplied thereto. For the purposes of description, a terminal used hereinmay be used to include both a general legacy terminal and an MTCterminal

First, a novel PRACH configuring method for supporting acoverage-limited terminal according to an embodiment of the presentinvention will be described. According to an embodiment of the presentinvention, a terminal required for coverage enhancement (hereinafter,referred to as a coverage-limited terminal) may transmit a PRACH using along preamble format in which a length of a preamble is determined onthe basis of a baseline RACH sequence. For example, a preamble format inwhich the baseline RACH sequence is repeated with the long preambleformat twice may be used as a second preamble format 2 for a PRACH ofthe coverage-limited terminal. In this case, in the long preambleformat, the baseline RACH sequence may be repeated 16 times for about 12to 15 TTI.

Meanwhile, in a case in which the long preamble format in which thebaseline RACH sequence is repeated is used, an RACH configuration period(e.g., 20 msec) of an existing terminal may not be valid. In the case ofusing the long preamble format, it has a long duration. For example, ina case in which an MTC terminal uses a long preamble format and a legacyterminal uses an existing preamble, short and long preambles maycoexist. In this case, a PRACH of the MTC terminal and a PRACH of thelegacy terminal may collide. In order for two PRACHs having differentformats to coexist without collision in uplink transmission, in anembodiment of the present invention, a new PRACH configuration index maybe defined. Such a new PRACH configuration may also be applied even to acase in which a preamble format is not newly defined. When it is assumedthat an existing preamble is repeatedly transmitted, it may be assumedthat the existing preamble is repeatedly transmitted only with resourceset as a PRACH configuration index and time.

The newly defined PRACH configuration index may be a PRACH configurationindex having a period longer than 20 msec. For example, setting of 160msec with respect to PRACH may be defined as illustrated in Table 13.Table 13 is a table illustrating a newly configured PRACH configurationindex.

TABLE 13 PRACH configuration Preamble System Frame Subframe index FormatNumber) Number 64 5 (or new (SFN % 16) = 0 0, 80 format)

The information regarding the new PRACH configuration index may betransmitted to the terminal through an SIB. An individual SIB may betransmitted to the terminal using a long preamble format. For example,the individual SIB may be transmitted to an MTC terminal suffering acoverage problem. Thus, in the present invention, as the BS transmitsthe new preamble configuration index to the terminal, the terminal maybe supported to use the new long preamble format to enhance coverage.Such a new index may also be applied even when an existing format isrepeatedly transmitted.

Meanwhile, together with the PSS/SSS, the terminal may perform trackingon a downlink channel transmitted by the BSs by using a cell-specific RS(CRS). In case of a terminal having a coverage problem, the terminal maynot properly perform channel estimation/tracking on the basis of theCRS. Hereinafter, a method for enhancing coverage using a CRS to solvesuch a problem will be described.

FIG. 18 is a conceptual view illustrating a method of configuring a CRSaccording to an embodiment of the present invention.

Referring to FIG. 18, a PDSCH may perform additional power boosting on aCRS of an allocated PRB. A PDSCH transmitted to a terminal (for example,when a channel transmission scheme is different from an existing case ina coverage issue) executing a coverage expansion mode is limited toboosting power with respect to a CRS corresponding to allocated PRB. R0is an RE in which a CRS regarding a first antenna port is transmitted,R1 is an RE in which a CRS regarding a second antenna port istransmitted, R2 is an RE in which a CRS regarding a third antenna portis transmitted, and R3 is an RE in which a CRS regarding a fourthantenna port is transmitted. By performing additional power boosting onthe CRSs transmitted in the 6RB (central 6RB) of R0 to R5 andtransmitting the same, the coverage-limited terminal may be supported toperform tracking regarding a downlink channel on the basis of the CRSs.

Here, power boosting may include power boosting regarding an additionalCRS of the PRB to which a PDSCH has been additionally allocated. Forexample, in consideration of the fact that the MTC terminal performs anarrow band operation, power boosting regarding a CRS transmitted in asubband region (e.g., central 6RB frequency band) that the MTC terminalmay receive may be performed.

Here, in a case in which additional power boosting (e.g., boostingexceeding 3 dB) is used to support the coverage-limited terminal, aninfluence of the additional power boosting on the legacy terminal shouldbe minimized To this end, in case of performing additional powerboosting, the following methods may be considered in order to minimizean influence on the legacy terminal

1) For additional power boosting, only a multimedia broadcast singlefrequency network (MBSFN) subframe may be used.

The MBSFN subframe, a subframe for transmitting a physical multicastchannel (PMCH), may be a subframe in which a boosted CRS is transmittedwithout affecting the legacy terminal in a region other than the PDCCHregion composed of first two OFDM symbols. Here, the CRS refers to areference signal that every terminal within a cell may recognize. Powerboosting (e.g., 9 dB power boosting) may be performed on symbolsexcluding two OFDM symbols that may be demodulated by the legacyterminal. The coverage-limited terminal may perform tracking on the BSon the basis of the power-boosted signal and/or channel transmitted inthe other symbols of the MBSFN subframe. To this end, it may be assumedthat the CRS is transmitted in the MBSFN for the terminal in thecoverage expansion mode.

2) In a resource domain in which the reference signal using additionalpower boosting is transmitted, QAM (quadrature amplitudemodulation)-modulated data may not be transmitted.

In order not to interfere with RRM (radio resource management)measurement, overall power of reference symbols used for RSRP may beequal or a subframe used in performing RRM measurement may be limited.The RRM measurement may include measurement of RSRP (reference signalreceived power) and RSRQ (reference signal received quality). The RSRPmay be power received for the CRS and the RSRQ may be quality of asignal calculated on the basis of the CRS.

When a reference signal used for the coverage-limited terminal toperform tacking is boosted, power of the other remaining RSs may berelatively reduced, whereby overall power of the reference symbols usedfor the RSRP may become equal to maintain the same RSRP, thus preventinginterfering RRM measurement. In this case, power of the other remainingreference symbols, excluding the reference signal used for thecoverage-limited terminal to perform tracking, may be reduced, and thus,a greater amount of error may occur when QAM modulation having arelatively high code rate is used. Thus, the QAM-modulated data may belimited not to be transmitted in the resource domain in which thereference signal using additional power boosting is transmitted. Forexample, when a reference signal is boosted in a particular subframe,the terminal may determine that QAM-modulated data is not transmitted ina power-boosted subframe. That is, in consideration of occurrence of anerror, it includes transmission of data on which a phase and anamplitude, that is, I channel and Q channel modulation has not beenperformed.

As well as the method of power boosting for the CRS, trackingperformance of the coverage-limited terminal may be enhanced byallocating an additional resource element to the CRS. Transmission ofthe additional CRS may be advantageous in that it does not greatlyaffect the legacy terminal

FIG. 19 is a conceptual view illustrating a method of configuring a CRSaccording to an embodiment of the present invention.

Referring to FIG. 19, it may be configured such that more CRSs than thelegacy CRS configuration are transmitted with respect to a frequencyband in which the coverage-limited terminal is transmitted. Bytransmitting additional CRSs 1900, tracking performance of thecoverage-limited terminal may be enhanced. This includes additionaltransmission of the CRSs only in particular PRBs. In particular, thePRBs in which the CRSs are added may be PRBs (for example, central 6RBs)demodulated by the coverage-limited MTC terminal. In another example,the PRBs may be PRBs included in an MBSFN subframe read by thecoverage-limited MTC terminal. Alternatively, in a case where a PDSCH istransmitted to a terminal in a coverage expansion mode, the PRBs may belimited to PRBs in which the corresponding PDSCH is transmitted.

1) The additional CRS may be transmitted in a resource element allocatedto a DM-RS in a sub-band. A region in which the additional CRS insteadof the DM-RS is transmitted may be a narrowband frequency band allocatedto the coverage-limited terminal or a PDSCH allocated to thecoverage-limited terminal

2) CRSs are additionally transmitted in OSDM symbols 2 and 3 of eachslot.

The additional CRSs may be transmitted in the OFDM symbols 2 and 3having the same frequency band as the frequency band in which the CRSsare transmitted in OFDM symbols 0 and 1.

A band in which the CRSs are additionally transmitted in the OFDMsymbols 2 and 3 may be a frequency band allocated to thecoverage-limited terminal or a frequency band in which a PDSCH allocatedto the coverage-limited terminal exists. For example, if the antennaport 0 is used as a single antenna port in the CRS transmission,positions of resource elements for the CRSs transmitted in other antennaport, the same as positions of CRSs transmitted in the antenna port 1and 2/3, may be used as additional CRS resource elements. If it isrequired to transmit a larger number of CRSs, CRSs may be additionallyused also in resource elements corresponding to other OFDM symbols andsubcarriers.

3) Resource elements corresponding to Vshift_new=Vshift_org+1 (or k) areused as CRSs

In another method, an additional CRS may be provided to thecoverage-limited terminal by transmitting other aggregation of CRSsusing 1 or k-increased Vshift. Vshift may be a variable for changing aposition in which a CRS is transmitted. For example, in a case where aCRS corresponding to Vshift_org=0 is used for initial CRS transmission,Vshift_new=2 may be used for transmitting a second aggregation. If otheraggregation is required, a CRS aggregation corresponding to Vshift_new=4may be transmitted as additional CRSs. Similarly, by defining Hshift,transmitted CRSs may be determined to be different according topositions of OFDM symbols in which CRSs are transmitted. For example, incase of Hshift=2, second aggregation of CRSs may be transmitted througha frequency band in which the CRSs are transmitted in OFDM symbols 1 and2, in OFDM symbols 2 and 3 of each slot. Positions of additionallytransmitted CRSs may be determined by using both Vshiaft_new and Hshift.That is, a position of an OFDM symbol in which the CRSs are transmittedand an aggregation of CRSs transmitted in a corresponding symbol may bedetermined A second aggregation of CRSs may be positioned in OFDMsymbols 2 and 3 on the basis of Vshift_new=Vshift_org+2 and a thirdaggregation of CRSs may be positioned in OFDM symbols 4 and 5 on thebasis of Vshift_new=Vshift_org+4 and transmitted.

(4) In a case where power boosting is performed on the coverage-limitedterminal, an influence on the legacy terminal and an MTC terminal whichis not required to enhance coverage needs to be reduced. Thus,additional power boosting may be limited to additional CRS resourceelements with respect to the coverage-limited terminal. When powerboosting is used for the entire frequency band of the coverage-limitedterminal using a narrow band, a power boosting rate in consideration ofthe additional power boosting may be transmitted to the terminal and theterminal may execute appropriate QAM demodulation. As well as CRSs, evena demodulation reference signal (DM-RS) used for demodulating a PDSCHmay also be newly set for the coverage-limited terminal

Meanwhile, in order to enhance demodulation performance on a DM-RStransmitted to the coverage-limited terminal, the following method maybe used. The DM-RS may be a reference signal for demodulating a PDSCHand a reference signal for demodulating a PUSCH. Hereinafter, anembodiment of the present invention may be applied to both a downlinkDM-RS and an uplink DM-RS.

In addition, an additional resource element may be selected to transmita redundant DM-RS. In addition, when additional power boosting than thecurrently defined power boosting (e.g., 3 dB) is considered for a DM-RS,an additional DM-RS may be transmitted as follows. First, for thepurposes of description, a DM-RS with respect to a PDSCH transmittedthrough downlink may be described as follows.

In a case where a single-layer transmission with respect to thecoverage-limited terminal is used, a first aggregation of DM-RSs mayinclude DM-RS resource elements regarding a normal subframe transmittedin an antenna port 7. When a normal DM-RS is used, for example, 12resource elements per PRB may be used.

If density of the DM-RSs is doubled, DM-RSs transmitted for a specialsubframe configurations 3 and 4 or 8 may be additionally used in DM-RSstransmitted on the basis of antenna port 7. In this case, 12 DM-RSs maybe additionally transmitted in the OFDM symbols 2 and 3 in each slot.

If density of the DM-RSs increases to four times, a first aggregation(DM-RSs transmitted in an antenna port 7 of a normal subframe andspecial subframe configurations 3, 4, or 8) and a second aggregation(the first aggregation and DM-RSs transmitted in antenna port 9 in anormal subframe) may be used to transmit a demodulation referencesignal.

When an extended CP is used (in case of an extended subframe, ratherthan a normal subframe), additional resource elements of DM-RSs may bepositioned in OFDM symbols 1 and 2 of each slot. An additional RS withrespect to the extended subframe collide with CRSs, and thus, resourceelement positions of the DM-RSs may be changed on the basis of Vshift.For example, a CRS may be transmitted in a resource element in whichVshift is set to 0, and a frequency band in which a DM-RS defined in anormal subframe, rather than in an extended subframe, is transmitted maybe used for transmitting the DM-RSs. For example, subcarriers 2, 5, 8,and 11 for a first slot and subcarriers 3, 6, 9, and 12 for a secondslot may be used to transmit the DM-RSs in the extended subframe.

As described above, with respect to the coverage-limited terminal, boththe CRS configuration and DM-RS configuration may be used or the CRSconfiguration and DM-RS configuration may be selectively used. In orderto enhance modulation performance regarding CRS and DM-RS transmitted tothe coverage-limited terminal, the following method may be additionallyused.

In a case where an MTC terminal requesting coverage enhancement operatesin a narrow bandwidth (e.g., 6PRB), an EPDCCH, rather than PDCCH, may beused for transmitting a control channel. In this case, an EPDCCH or aPDCCH based on a CRS or a DM-RS may be transmitted additionally in a CRS(or TRS in a cell (e.g., NCT) in which reference signals are transmittedin a reduced form). According to an embodiment of the present invention,in order to enhance channel estimation performance, a CRS (or a TRS) anda DM-RS may be simultaneously used for channel estimation. The CRS (orTRS) and the DM-RS may be simultaneously used for channel estimationthrough various implementations.

In order to use a CRS (or a TRS) and a DM-RS for channel estimation, theterminal may need to know a precoding matrix used in the DM-RS. Thus,information for indicating a precoding matrix used in the DM-RS or apredetermined precoding may be transmitted through higher layersignaling. In another method, precoding for a DM-RS may not be used orpredetermined precoding may be assumed.

Whether a terminal may simultaneously use a CRS (or a TRS) and a DM-RSmay be indicated by using higher layer signaling or an indicator of anMIB (PBCH) or an SIB. When the indicator indicating whether the terminalsimultaneously uses a CRS (or a TRS) and a DM-RS is turned on, it may beassumed that the terminal uses precoding for the DM-RS or predeterminedprecoding is used for DM-RS.

Higher layer signaling may be used to change a precoding matrix or allowfor precoding for a DM-RS. In addition to precoding used for DM-RS, theterminal may assume that an antenna port used for CRS (or TRS) and anantenna port used by the DM-RS are the same such that channels are thesame.

According to another embodiment of the present invention, a widefrequency bandwidth for an RS may be used, compared with a PDSCHfrequency band.

In order to increase density of RSs, a frequency bandwidth wider than afrequency bandwidth in which a PDSCH is transmitted may be used for RStransmission. For example, when a BS supports a 10 Mhz system bandwidthand an MTC terminal requiring coverage enhancement operates in 1.4 Mhzsystem bandwidth, the terminal may perform channel estimation on 3 Mhz,instead of 1.4 Mhz in order to enhance channel estimation performance.This method may not be used for a narrowband carrier (e.g., 1.4 Mhz),and since the terminal should know a maximum system bandwidth, a PBCH,an SIB, or higher layer signaling may be given to indicate a systembandwidth of a carrier.

Downlink data of different configurations may be received according todegrees of coverage enhancement required by the terminal Hereinafter, amethod for enhancing coverage of terminals each requesting differentcoverage enhancement will be described in an embodiment of the presentinvention.

FIG. 20 is a conceptual view illustrating a method of transmitting andreceiving data by a coverage-limited terminal according to an embodimentof the present invention.

Referring to FIG. 20, coverage-limited terminals requiring coverageenhancement may request different degrees of coverage enhancement from aBS 2000. For example, degrees of enhancement requested bycoverage-limited terminals may be different according to locations ofthe coverage-limited terminals. For example, a first terminal 2010 mayrequire enhancement of strength of a downlink signal of 0 to 5 dB, asecond terminal 2020 may require enhancement of strength of a downlinksignal of 5 to 10 dB, a third terminal 2030 may require enhancement ofstrength of a downlink signal of 10 to 15 dB, and a fourth terminal 2040may require enhancement of strength of a downlink signal of 15 to 20 dB.

In a case where a greater coverage enhancement is required, greateroverhead may be required for data transmitted by the terminal Thus, adegree of coverage enhancement may be adaptively determined on the basisof information regarding how great enhancement is required for eachterminal To this end, a broadcast channel such as a PBCH may beconfigured to be transmitted on the assumption of a terminal whichdesires coverage enhancement to the maximum so as to be resistant anerror. However, a unicast channel with respect to a particular terminalsuch as a PDSCH may adaptively perform an operation required forcoverage enhancement on the basis of a degree of coverage enhancementrequired for each terminal.

In order to determine information regarding coverage enhancementrequired for a terminal, various types of information may be used. Forexample, information regarding coverage enhancement required for aterminal may be determined on the basis of (1) information regarding thenumber of PSS/SSSs received by the terminal for decoding the PSS/SSSs,(2) information regarding an average SNR received by the terminal suchas RSRP, (3) information regarding an average number of PBCHs receivedby the terminal for decoding an MIB, and (4) BS determinationinformation based on RACH reception.

When (1), (2), and (3) are used, required coverage-enhancementinformation may be signaled from the terminal to the BS. Thus, accuratecoverage-enhancement may be used to transmit a PDSCH or an RS from theBS to the terminal

In an embodiment of the present invention, an operation of a BSperformed according to a coverage enhancement request required by acoverage-limited terminal is proposed. For example, it may includegrouping coverage enhancement requested by a terminal to a predeterminedrange and adaptively performing an operation of the BS according to acoverage enhancement request group. The BS may transmit data to theterminal by using different RS transmission densities, differentrepeated transmissions and repeated number, different MCSs, and thelike, according to each coverage enhancement request group.

An example is illustrated in Table 14 below. A group of terminals forrequesting coverage enhancement of the terminals may be transmitted tothe terminals through higher layer signaling. When terminals fail toreceive information regarding a group thereof for requesting coverageenhancement through higher layer signaling, a coverage enhancementrequest may be made by setting a coverage enhancement group of theterminals to a maximum coverage enhancement request group. For example,the maximum coverage enhancement request group may be set to a defaultvalue.

TABLE 14 RACH TTI Requirement RS density Repetition Bundling 0-5 dB Once(normal subframe port 7) 2 4 5-10 dB Twice (normal subframe port 7 + 432 special subframe conf 3 port 7) 10-15 dB Four times (normal subframe8 ~100 port 7/8 + special subframe conf 3 port 7/8) 15-20 dB Four times(normal subframe 16 ~300 port 7/8 + special subframe conf 3 port 7/8)

Referring to Table 14, in the case of the first coverage enhancementrequest group (0 to 5 dB), density of the reference signals (DM-RSs) ofthe BS transmitted to the coverage-limited terminal may be the same asthat of the existing case. The terminal may transmit a generated PRACHby repeating a sequence twice. Also, the terminal may transmitcontinuous data having four different redundant versions which haveperformed TTI bundling to the BS. The TTI bundling may also be appliedto a downlink channel.

In the case of the second coverage enhancement request group (5 to 10dB), density of the reference signals (DM-RSs) of the BS transmitted tothe coverage-limited terminal may be two times the existing case andtransmitted. When transmitting a PRACH, the terminal may transmit thegenerated PRACH by repeating a sequence four times. Also, the terminalmay transmit continuous data having 32 different redundant versionswhich have performed TTI bundling to the BS.

In the case of the third coverage enhancement request group (10 to 15dB), density of the reference signals (DM-RSs) of the BS transmitted tothe coverage-limited terminal may be four times the existing case andtransmitted. When transmitting a PRACH, the terminal may transmit thegenerated PRACH by repeating a sequence eight times. Also, the terminalmay transmit continuous data having 100 different redundant versionswhich have performed TTI bundling to the BS.

In the case of the fourth coverage enhancement request group (15 to 20dB), density of the reference signals (DM-RSs) of the BS transmitted tothe coverage-limited terminal may be four times the existing case andtransmitted. When transmitting a PRACH, the terminal may transmit thegenerated PRACH by repeating a sequence 16 times. Also, the terminal maytransmit continuous data having 300 different redundant versions whichhave performed TTI bundling to the BS.

Referring to Table 14 described above, in another example of the presentinvention, different groupings may be performed according to a requestfrom a coverage-limited terminal, and operations of the BS and theterminal may be controlled to be performed accordingly. Thus, the scopeof the present invention is not limited to Table 14. Also, according toanother example of the present invention, the coverage extension levelrequest may be set by the BS, and after the coverage extension levelrequest is set, it may be used as a method for designating with which RSdensity and TTI bundling number it should operate by the terminal

Meanwhile, a case in which an MBSFN subframe is used to performcommunication between the coverage-limited terminal and the BS, or acase in which an NC subframe is used to perform communication betweenthe coverage-limited terminal and the BS may also be assumed. In thiscase, the coverage-limited terminal may not perform sufficient trackingbased on a normal CRS or TRS. Thus, an additional RS for tracking mayneed to be defined. Thus, an additional RS for tracking may be definedas a reference signal transmitted by increasing density of CRSs or TRSshigher than that of the existing case, thus enhancing trackingperformance.

In another method, tracking performance of the terminal may be enhancedon the basis of a DM-RS. According to an example of the presentinvention, when a DM-RS-based method is used, the DM-RS may be used as agroup-specific RS, rather than as a terminal-specific RS. That is,referring to Table 14, for example, a group-specified RS may be appliedaccording to a coverage enhancement request group. Here, according to anexample of the present invention, different power boosting may beapplied to the RS according to the coverage enhancement request group,or a repeated RS transmission may be included. for example, a pluralityof coverage-limited terminals may share the same DM-RS patternregardless of whether a terminal-specific PDSCH exists.

For example, a case in which central 6RBs are allocated tocoverage-limited terminals in an MBSFN subframe may be assumed. Eachcoverage-limited terminal may receive a DM-RS in which nscid=0 or 1regardless of a value of nscid defined in a DCI format 2B or 2C (ifused) of the central 6RBs of the MBSFN subframe, thereby assuming aDM-RS of the same pattern. In a case where density of DM-RSs is theworst case, the terminals share the DM-RS of the same pattern regardlessof whether a PDSCH exists, thereby performing tracking on a downlinkchannel with respect to the BS.

According to an embodiment of the present invention, for a particularterminal, a short PBCH excluding predetermined information may betransmitted with respect to a PBCH transmitted to the legacy terminal.In the existing case, a MIB (master information block) or an SIB (systeminformation block) may be used to transmit system information. The MIBincludes a downlink cell bandwidth, a system frame number, and the like,and may be transmitted through a PBCH. The SIB may be defined bySIB0˜SIB10 included therein and used. The SIB may be transmitted througha PDSCH.

In particular, an MTC terminal having a narrow bandwidth may not requirepartial information (e.g., system bandwidth and/or PHICH configurationinformation) among information transmitted by the PBCH. Thus, coverageof the terminal may be enhanced by increasing a coding gain byincreasing coding efficiency on the basis of very short PBCH omittingunnecessary information.

In a case where a subframe transmitting the MIB is fixed to a subframecorresponding to 0 or 0/5 with respect to each radio frame, thecoverage-limited terminal may transmit the MIB through an (E)PDCCHand/or PDSCH similar to the SIB-1. In addition, the MIB and the SIB-1may be combined to include essential information which cannot bepreviously determined or hard-coded.

For example, a newly defined SIB (SIB 0) may be expressed as shown inTable 15. The new SIB may not transmit partial information with respectto a terminal (e.g., a coverage-limited terminal).

TABLE 15 SystemInformationBlockType0 ::=   SEQUENCE {  systemFrameNumberBIT STRING (SIZE (8)),  supportNarrowBandMTC ENUMERATED  {0,1} #0  fordisabled or #1 for enabled }

Referring to Table 15, only system frame number and informationregarding whether a particular terminal (e.g., a narrowband MTCterminal) are supported may be transmitted.

Alternatively, according to another example of the present invention, acombination of SIB 0 and SIB 1 may be considered in the form as shown inTable 16 below.

TABLE 16 SystemInformationBlockType0-1 ::= SEQUENCE  systemFrameNumberBIT STRING (SIZE (8)),  supportNarrowBandMTC ENUMERATED  {0,1} #0  fordisabled or #1 for enabled  cellAccessRelatedInfo SEQUENCE {  plmn-IdentityList PLMN-IdentityList,   trackingAreaCodeTrackingAreaCode,   cellIdentity CellIdentity,   cellBarredENUMERATED{barred, notBarred},   intraFreqReselection ENUMERATED{allowed, notAllowed},   },   cellSelectionInfo SEQUENCE {    q-RxLevMin Q-RxLevMin,   },   freqBandIndicator   INTEGER(1..64),  schedulingInfoList SchedulingInfoList,   tdd-Config TDD-ConfigOPTIONAL, -- Cond TDD    si-WindowLength ENUMERATED {ms1, ms2, ms5,ms10, ms15, ms20, ms40},   systemInfoValueTag INTEGER (0..31), }

If a PBCH is transmitted only four times at an interval of 10 msecwithin 40 msec, the coverage-limited terminal may not be able to detectthe PBCH within 40 ms. Thus, in the case of the coverage-limitedterminal, the PBCH may be transmitted more frequently than 10 msec suchthat the terminal may detect the PBCH within 40 ms or change an SFN bitto 7 bits. For example, 3 LSB among 7 bits may be detected by decodingof the PBCH, and an aggregation of PBCHs may be transmitted over 80msec.

Hereinafter, several methods for transmitting a redundant PBCH in theform including a new PBCH in addition to an existing PBCH and a repeatedPBCH with respect to the coverage-limited terminal will be described indetail.

FIG. 21 is a conceptual view illustrating a method of transmitting aPBCH according to an embodiment of the present invention.

Referring to FIG. 21, an additional redundant PBCH may be transmitted inan additional resource domain (central 6RB region) 2100 as well as in anexisting PBCH transmission region. The additional PBCH may betransmitted in the central 6RB among the OFDM symbols after the PDCCHallocation region of the subframe 0 to subframe 5. Here, the redundantPBCH may also use the entirety of a portion of OFDM symbols excludingthe existing PSS, SSS, and PBCH regions in the central 6RB region. Also,in a case where the DM-RS cannot be transmitted in a transmission mode 9and a transmission mode 10, the additional PBCH may be allocated even inthe corresponding resource region. Hereinafter, transmission of theadditional PBCH will be described in detail.

(1) Transmission of redundant PBCH: The PBCH may be repeatedlytransmitted in a different resource domain. For example, the same PBCHmay be repeatedly allocated in a first slot (or the other remaining OFDMsymbols not used as PSS/SSS/PDCCH by using OFDM symbols 1 to 4. Theredundant versions of the repeated transmitted PBCH may be differentlyset to enhance decoding performance when they are received by theterminal.

For example, in a case where a duplexing method in a first slot of asubframe #0 is FDD, the PBCH may be transmitted in the OFDM symbols 2,3, and 4, and in a case where the duplexing method is TDD, the PBCH maybe transmitted in OFDM symbols 5 and 6. Also, in a second slot ofsubframe #0, in case of the FDD mode, the PBCH is transmitted in OFDMsymbol 5, and in the TDD mode, the PBCH may be transmitted in OFDMsymbols 4 and 5. Similarly, the same PBCH may be repeatedly transmittedin the subframe #5. For example, different redundant PBCH may betransmitted in OFDM symbols 0 to 3 in a second slot, or a first slot orany other OFDM symbol. If necessary, additional redundant PBCHtransmission may be performed in mutually different subframes like asubframe #1 and a subframe #5.

(2) Additional transmission of new PBCH: For example, a PBCH withrespect to a particular terminal such as an MTC terminal may be newlydefined as an MTC-PBCH and used. In order to transmit a new PBCH withrespect to an MTC terminal, a redundant MTC-PBCH may be transmitted inthe same position as that of the method of 1). Here, the MTC-PBCH iscomposed of small bits compared with a normal PBCH, and thus, two OFDMsymbols may be used to transmit the MTC-PBCH, instead of using four OFDMsymbols. Also, in a single subframe #0, three MBC-PBCH may betransmitted, and five MTC-PBCHs may be transmitted in other subframe.For example, the MTC-PBCHs may be transmitted by using OFDM symbols 2and 3 in a first slot, 4+4 OFDM symbols in each slot, and 0-1/2-3/5-6 ina second slot with respect to FDD.

In addition, in case of using a new PBCH or an additionally repeatedPBCH, the new PBCH or the repeated PBCH may be transmitted through theentire OFDM symbols, excluding OFDM symbols used for a PDCCH, or may betransmitted through 3 to 13 OFDM symbols (normal CP) or 3 to 11 OFDMsymbols (extended CP). Here, a bandwidth or the number of entirecarriers used for PBCH transmission may be limited to a small number ofPRBs such as 1 or 2 PRBs. Here, a repeated SIB may be generally appliedfor the coverage-limited terminal. Here, the OFDM symbols used for thePBCH may include OFDM symbols for transmitting the PBCH using additionalpower or OFDM symbols multiplexed with other signals for supportingother UEs including an existing latent legacy terminals and transmitted.Here, the PBCH may include a PBCH extending to the entire subframes orthe OFDM symbols, and repeated PBCH may be implemented, and here, whenPSS/SSS is not transmitted, a final symbol may be used to transmit thePBCH.

As described above, the number of PRBs for the PBCH may be limited toone or two PRBs. Here, power for transmitting the PBCH, the same as thepower used for transmitting six PRBs, may be used. That is, when thePBCH is transmitted through 1 PRB, 6-fold power boosting (×6 times, (7dB)) is applied, or when the PBCH is transmitted through the 2PRB,3-fold power boosting (×3 times, (4 dB)) may be applied. Here, in a casewhere the power boosting is applied in the 6 PRBs allocated for the PBCHtransmission, any other data, excluding 1 or 2 PRBs used fortransmission of the PBCH and/or CRS for supporting the power boosting,may not be transmitted.

Meanwhile, the two following techniques may be considered to transmit aredundant PBCH. One is a circular repetition, and the other is a simplerepetition technique.

FIG. 22 is a conceptual view illustrating a method of transmitting asingle MIB through a PBCH according to an embodiment of the presentinvention.

In FIG. 2, a method of transmitting one MIB through a PBCH is disclosed.One MIB may include 14 information bits and 10 reserved bits. In orderto transmit one MIB, a CRC (16 bits) may be additionally attached.Coding and rate matching may be performed on PBCH data with availablebits. Coding and rate matching-performed bits may be divided into fourunits that can be decoded individually. According to an embodiment ofthe present invention, four divided information units may be transmittedthrough the circular repetition or the simple repetition.

FIG. 23 is a conceptual view illustrating a method of transmitting aPBCH using a circular repetition method according to an embodiment ofthe present invention.

Referring to FIG. 23, in case of using the circular repetition method,the MTC-PBCH or the legacy PBCH may be repeatedly transmitted in a radioframe on the basis of the circular repetition method. For example, asillustrated in NFIG. 22, one MIB is coded and divided into four codeblocks. For example, in every radio frame, repetitions may be performedtwice in the subframe #0, and other repetitions may be performed twicein subframe #5. The code blocks 1/2/3/4 may be transmitted in thecircular repetition method as illustrated in FIG. 23. Four code blocksobtained by dividing one MIB may be transmitted on the basis ofdifferent allocation methods in a plurality of radio frames.

For example, in the first radio frame 2310, a first code block and asecond code block may be transmitted in subframe #0 (2310-0) and thirdcode block and a fourth code block may be transmitted in a subframe #5(2310-5). In the second radio frame 2320, the third code block and thesecond code block may be transmitted in the subframe #0 (2320-0) and thefirst code block and the fourth code block may be transmitted in thesubframe #5 (2320-5). That is, four code blocks may be circulatively andrepeatedly transmitted in different resource allocation positions indifferent radio frames

FIG. 24 is a conceptual view illustrating a method of transmitting aPBCH using simple repetition according to an embodiment of the presentinvention.

Referring to FIG. 24, in case of using the simple repetition method, anMTC-PBCH or a legacy PBCH may be repeated in a radio frame. The circularrepetition method may be repeatedly executed in a single radio frame.For example, one MIB may be coded and may be divided into four codeblocks as illustrated in FIG. 22. It may be assumed that repetition isperformed twice in the subframe #0 and other repetition is performedtwice in subframe #5. The same code blocks of the radio frame may betransmitted as illustrated in FIG. 24.

Among the four code blocks, a first code block may be transmitted twiceeach in the subframe #0 (2410-0) and subframe #5 (2410-5) included inthe first radio frame 2410, and thus, the first code block may betransmitted repeatedly a total of four times. Among the four codeblocks, the second code block may be transmitted twice each in thesubframe #0 (2420-0) and subframe #5 (2420-5) included in the secondradio frame 2420, and thus, the second code block may be transmittedrepeatedly a total of four times. The third code block and the fourthcode block may be repeatedly transmitted in the same manner. In thismanner, one code block may be transmitted repeatedly in one radio frame.

Hereinafter, a method of adaptively transmitting a PBCH and otherdownlink channel by a BS with respect to various levels (0 to 5 dB, 5 to10 dB, etc.) requesting coverage enhancement (handling different levelsof coverage enhancements) according to an embodiment of the presentinvention will be described. Here, a coverage-limited terminal maydetermine coverage enhancement request information thereof by measuringor reading an acquisition time of a PSS/SSS or a detect signal.

For example, in order to request coverage enhancement, thecoverage-limited terminal may request an operation of the BS forcoverage enhancement with respect to four different levels (e.g., 0 dB,5 dB, 10 dB, and 20 dB). For example, with respect to repetition of aPBCH, in a case where coverage request information of the terminal is 0dB, it may be assumed that non-repetition is requested because legacyPBCH is sufficient. In a case where coverage request information of theterminal is 5 dB, it may mean that repetition of the PBCH is requestedtwice from the BS (for example, 8 PBCHs for 40 ms). In a case wherecoverage request information of the terminal is 10 dB, it may mean thatrepetition of the PBCH is requested four times from the BS (for example,16 PBCHs for 40 ms). In a case where coverage request information of theterminal is 20 dB, it may mean that repetition of the PBCH is requested40 times from the BS (for example, 40 PBCHs for 40 ms).

A radio frame for transmitting a PBCH with respect to thecoverage-limited terminal needs to be determined. If not, thecoverage-limited terminal may not be able to successfully detect a PBCHtransmitted by the BS. The radio frame in which the PBCH is transmittedmay be determined on the basis of coverage enhancement requestinformation requested by the terminal. For example, a redundant PBCH maybe transmitted at every five minutes, based on which the MTC terminalmay receive the PBCH. The PBCH may be transmitted at the interval offive seconds for 40 ms with respect to a predetermined time (forexample, 0 hour (midnight)). The PBCH is transmitted for 40 ms withrespect to 0 h 0 m 0 s, and a redundant PBCH may be transmitted for 40ms with respect to 0 h 0 m 5 s. A transmission level of the redundantPBCH according to coverage enhancement may be classified as follows.

(1) In a case where four PBCHs (legacy PBCHs) are transmitted for 40 ms,it may be a redundant PBCH level 1. In the redundant PBCH level 1, thePBCHs may be allocated in the positions where the existing legacy PBCHsare transmitted, and transmitted to the terminal. The four PBCHs may beL PBCH1, L PBCH2, L PBCH3, and L PBCH4, respectively.

(2) in a case where five PBCHs are required to be repeated with respectto a 5 dB coverage enhancement request, a position in which a secondaggregation of the PBCHs is to be transmitted may be previouslydetermined or may be set by a higher layer. For example, subframe 1 of aradio frame may be used to transmit the second aggregation of PBCHs tothe terminal in the same OFDM symbol in the BS.

In TDD, OFDM symbols 0 to 4 of a first slot may be used to transmit aredundant PBCH. Alternatively, in TDD, a required period may be assumedfor terminals limited in redundant PBCH transmission, and every subframeor a subframe transmitting a redundant PBCH may be used as a downlinksubframe, rather than an uplink subframe. A legacy terminal cannot beschedule in the subframe switched from uplink to downlink.

For example, a redundant PBCH transmitting the same content in L PBCH1(subframe #1 of a first radio frame), L PBCH2 (subframe #1 of a secondradio frame), L PBCH3 (subframe #1 of a third radio frame), and L PBCH4(subframe #1 of a fourth radio frame) may be defined. The terminal maydetermine whether a repeatedly transmitted PBCH exists on the basis ofthe PBCH transmitted in the subframe #1. A terminal requesting 5 dBcoverage enhancement may stop receiving of the PBCH. That is, theterminal may not receive an additional PBCH transmitted to a terminalrequesting 10 dB or 20 dB, receive only a PBCH with respect to a 5 dBcoverage request, and stop PBCH demodulation.

(3) With respect to a 10 dB coverage enhancement request (in a casewhere the BS determines to support), a new position of an additionalPBCH as in (2) may be selected. For example, subframes 2 and 3 may beused for retransmission. The new position of the additional PBCH may bedetermined in advance. The accurately same PBCH may be transmitted inthe new position. The terminal may detect whether an additional PBCH istransmitted by comparing such a signal with a legacy PBCH signal. Here,in order to allow for an error, detection of a signal may be performedsimilarly to a PSS detection based on correlation. When a redundant PBCHis required in TDD, an uplink subframe may be used as downlink similarto the 5 dB coverage request.

(4) For a 20 dB coverage enhancement request, the process may beperformed on the basis of the procedure similar to (3).

A new position of the PBCH with respect to each level (5 dB, 10 dB, 15dB, 20 dB) of coverage enhancement may be selected from other radioframe outside of the boundary of 40 msec. For example, a legacy PBCH maybe transmitted from SFN=0 to 3. Also, a first aggregation of redundantPBCHs may be transmitted in SFN=4 to 7. In this case, a position of anadditional PBCH may be different from the legacy position in order toreduce an influence on the legacy terminal which does not know about thecoverage enhancement. For example, a PBCH for the PBCH coverage-limitedterminal may be transmitted through 0 to 4 OFDM symbols in a secondslot.

According to the present invention, the BS may select a coverageenhancement level with respect to a system layout and a latent terminal,and thus, overhead may be reduced. In other words, a position of thecorresponding redundant PBCH may be fixed to correspond to the coverageenhancement level and transmitted from the BS to the terminal Thus, theBS may perform an operation according to the coverage enhancementrequest on the basis of the network layout and the latent coverageenhancement request. Also, the terminal may detect the PBCH in apredetermined position on the basis of the coverage request matters ofthe terminal.

Also, according to an embodiment of the present invention, a randomaccess preamble used by the coverage-limited terminal may be specifiedto be used. With respect to the coverage-limited terminal, atransmission coverage of Msg 3 (RRC connection request message) may be aburden in terms of uplink transmission. The reason is because it isrequired to repeatedly transmit Msg3. Thus, the coverage-limitedterminal may perform contention-free random access in performing randomaccess. In order for the coverage-limited terminal to performcontention-free random access, paging with respect to thecoverage-limited terminal may include a preamble sequence. As thecoverage-limited terminal uses a particular preamble sequence in therandom access, the coverage-limited terminal may perform contention-freerandom access without performing contention. In a case where thecoverage-limited terminal performs an initial setting, paging may not beset, and thus, the terminal may still need to perform thecontention-based random access procedure.

Also, according to an embodiment of the present invention, in a casewhere PDSCH transmission is performed through TTI bundling, frequencydiversity may be used to maximize a diversity gain. As a simple methodfor enabling PDSCH frequency diversity, resource is frequency-hoppedaccording to a predetermined pattern or the PDCCH transmits information(for example, offset) regarding a frequency hopping pattern to be usedfor next frequency hopping resource. Here, the offset may be usedaccording to DVRB (distributed virtual resource block) in which a VRB(virtual resource block) index is used instead of a PRB (physicalresource block) index to apply the offset to the VRB.

Also, according to an embodiment of the present invention, in order toenhance coverage of PSS/SSS, transmission of a plurality of PSS/SSSsfrom the BS to the coverage-limited terminal may be considered. In acase where the additional PSS/SSSs are transmitted, a symbol gap betweenPSS/SSSs and/or a subframe gap between two continuous PSS/SSS pairs maybe set, and accordingly, the legacy terminal may be prevented fromdetecting the additional PSS/SSSs. An example of position of theadditional PSS/SSS is as follows.

(1) An OFDM symbol gap between the PSS/SSS may not be 1 with respect toFDD, and may not be limited to −4 with respect to TDD. That is, asubframe gap between the PSS/SSSa may be 2, 3, and 4 in FDD and may be−1 or −2 in TDD. In the present invention, inclusion of other valuesthan the example values is not excluded.

(2) a subframe gap between two continuous PSS/SSS is not limited to 5.That is, two continuous PSS/SSS pars of additional PSS/SSSs may be 1, 2,3, 6, 7, 8, and 10.

Here, (1) and/or (2) may be used to define positions of the additionalPSS/SSSS. If PSS/SSS repetition is required one or more times, the samevalue may be used as the gap, or a gap having a different value may beused to transmit the repeated PSS/SSSs.

FIG. 25 is a block diagram illustrating a wireless communication systemaccording to an embodiment of the present invention.

Referring to FIG. 25, a BS 2500 includes a processor 2510, a memory2520, and a radio frequency (RF) unit 2530. The memory 2520 is connectedto the processor 2510 and stores various information for driving theprocessor 2510. The RF unit 2520 is connected to the processor 2510 andreceives and/or receives a radio signal. The processor 2510 implementsthe functions, processes and/or methods of FIGS. 1 through 24. In theforegoing embodiment, the operation of the BS may be implemented by theprocessor 2510.

For example, the processor 2510 may be implemented to receive a coverageenhancement request from the terminal, determine a downlink transmissionconfiguration transmitted to the terminal on the basis of the coverageenhancement request, and transmit downlink data on the basis of thedownlink configuration. In particular, the processor 2510 mayselectively control power boosting with respect to additional CRCtransmission allocation (which includes the entire radio resourceincluding a frequency domain, a particular symbol, a subframe, and thelike), the entire symbols or a particular symbol, and configure a MBSFNsubframe therefor or control the use of a particular modulation scheme.This also includes selectively configuring a set with respect toreference signals in consideration of collision with a particularreference signal (DM-RS), and the like.

Also, according to the present invention, in order to transmit aparticular reference signal, namely, in order to enhance tracking forfrequency/time of measurement of a terminal, and cell identification,the processor 2510 may control transmission density of the correspondingreference signal according to a determined group or a variableselection. Also, in order to effectively transmit the reference signals,the processor 2510 may selectively define normal CP/extended CPs toconfigure a subframe in the terminal and may variably define a channelconfiguration. To this end, a variable PRACH or an additional/repeatedPBCH configuration with respect to a particular terminal may be set.Thus, in order to receive an additional/repeated PBCH by the terminal,as well as general controlling to perform the PRACH, a positionregarding the channel configuration (including configuration informationincluding resource information regarding a time/frequency domain) istransmitted to the terminal

A wireless device 2550 includes a processor 2560, a memory 2570, and anRF unit 2580. The memory 2570 is connected to the processor 2560 andstores various information for driving the processor 2560. The RF unit2580 is connected to the processor 2560 and receives and/or receives aradio signal. The processor 2560 implements the functions, processesand/or methods of FIGS. 1 through 24. In the foregoing embodiment, theoperation of the BS may be implemented by the processor 2560.

For example, the processor 2560 transmits capability information thereofor information including a group regarding a coverage enhancementrequest of the terminal determined by a BS, and the like, to the BS. Inthis manner, the processor 2560 may transmit the coverage enhancementrequest signal regarding capability thereof. Also, the processor 2560may receive a setting regarding a PRACH variably configured by the BSaccording to the coverage enhancement request and a configurationregarding TTI bundling regarding downlink data and control/referencesignals, and operate variably according to the set configurationinformation.

For example, the processor 2560 may measure a signal and transmitscapability information thereof or a coverage enhancement request of theterminal to the BS in order to request for a required signal accordingto the measured signal or in consideration of a service required by auser. Also, the processor 2560 receives configuration information of achannel (limited according to characteristics of the terminal) accordingto the coverage request. Here, the configuration information of thechannel includes PRACH/PBCH/PDSCH and information regarding referencesignals repeated/added in a predetermined subframe. Thus, the processor2560 performs the PRACH according to the configured channel setting orcontrols to receive downlink data. In particular, the processor 2560checks information regarding additional CRC transmission allocation(which includes the entire radio resource including a frequency domain,a particular symbol, a subframe, and the like), and the entire symbolsor a particular symbol, and receive selectively power-boosted channel orreceive a corresponding reference signal or a PBCH in an additionallyallocated region. Also, through an MBSFN subframe therefor, theprocessor 2560 may control the use of a particular modulation scheme andreceives the reference signals and channels. This includes selectivelychecking a set with respect to reference signals in consideration ofcollision with a particular reference signal (DM-RS), and the like, andreceives the corresponding signal in the region of the correspondingreference signals.

Thus, according to the present invention, in order to transmit aparticular reference signal, namely, in order to enhance tracking forfrequency/time of measurement of a terminal, and cell identification,the processor 2560 may receive a signal with increased transmissiondensity of the corresponding reference signal according to a determinedgroup or in a variable position. Here, in order to effectively transmitthe reference signals, the processor 2560 may selectively define normalCP/extended CPs to configure a corresponding subframe and may variablydefine a channel configuration to receive the same. That is, accordingto variable PRACH configuration and additional/repeated PBCHconfiguration with respect to a particular terminal, the processor 2560enhances efficiency of the corresponding service through resourceinformation regarding a time/frequency domain of a additional/repeatedPBCH, and also, by receiving a repeatedly transmitted PDSCH, as well asgeneral controlling to perform the PRACH.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

In the above exemplary systems, although the methods have been describedon the basis of the flowcharts using a series of the steps or blocks,the present invention is not limited to the sequence of the steps, andsome of the steps may be performed at different sequences from theremaining steps or may be performed simultaneously with the remainingsteps. Furthermore, those skilled in the art will understand that thesteps shown in the flowcharts are not exclusive and may include othersteps or one or more steps of the flowcharts may be deleted withoutaffecting the scope of the present invention.

What is claimed is:
 1. A method for transceiving a reference signal by aterminal in a wireless communication system, the method comprising:determining capacity information of the terminal; confirming informationon an added reference signal corresponding to the capacity information;and transceiving a corresponding reference signal at a predeterminedsymbol or a frequency domain using the information on an added referencesignal, wherein the added reference signal comprises at least one of acell specific reference signal (CRS), a demodulation reference signal(DM-RS) or an MBSFN-RS, and wherein the capacity information of theterminal comprises a coverage improvement request level confirming anintensity of a downlink signal by the terminal and selectedcorresponding to the intensity of the signal, or information indicatingthat the terminal is a machine type communication (MTC) terminal.
 2. Themethod of claim 1, wherein the transceiving of the correspondingreference signal comprises receiving the added RS through a centerresource block (RB) configured by resource elements (RE) of a determinedposition of a physical downlink shared channel (PDSCH) assigned for theterminal, and wherein the RS comprises a selective power boostingapplied reference signal.
 3. The method of claim 2, wherein thetransceiving of the corresponding reference signal comprises: receivinga power boosted CRS through symbols except for symbols assigned for aphysical downlink control channel (PDCCH) at a MBSFN sub-frame fortransmitting a physical multicast channel (PMCH), and wherein the CRScomprises data to which QAM modulation is not applied.
 4. The method ofclaim 2, wherein the transceiving of the corresponding reference signalcomprises: at least one of receiving an additional CRS from apredetermined region for a DM-RS, receiving an additional RS from secondand third symbols of each slot; or an additional CRS from a symboldetermined according to at least one parameter distinguished from aninitial CRS, and wherein the at least one parameter comprises a V shiftvalue indicating a group of the additional CRS and an H shift valueindicating a position a symbol of a corresponding group.
 5. The methodof claim 1, wherein the transceiving of the corresponding referencesignal comprises: receiving the added DM-RS at a predeterminedchangeable position according to the coverage enhancement request level,and the receiving of the added DM-RS comprises receiving the added DM-RSin an RE of a predetermined DM-RS for a specific antenna port of anormal sub-frame, and wherein a density of the additional DM-RS isincreased ever twice according to the coverage enhancement requestlevel.
 6. The method of claim 5, wherein the transceiving of thecorresponding reference signal further comprises receiving the addedDM-RS from an RE of a predetermined DM-RS for at least one antenna portaccording to setting of a special sub-frame.
 7. The method of claim 6,wherein the transceiving of the corresponding reference signal furthercomprises receiving the added DM-RS from a predetermined symbol of anextension sub-frame, and wherein a position of the added DM-RS ischanged by slots by applying a V shift value by taking intoconsideration collision with a CRS.
 8. The method of claim 1, furthercomprising: confirming that a transmission density is changeably setwith respect to at least one reference signal among an additional CRS,an additional DM-RS, an additional tracking reference signal (TRS), anda newly defined RS; and confirming that selective power boosting isapplied to the at least one reference signal.
 9. An apparatus fortransceiving a reference signal in a wireless communication system, theapparatus comprising: a Radio Frequency (RF) unit that sends andreceives signals to and from a base station; and a processor coupled tothe RF unit to process the signal, wherein the processor configured totransmit capacity information of a terminal to the base station, confirminformation on an added reference signal corresponding to the capacityinformation, and transceive a corresponding reference signal at apredetermined symbol or a frequency domain to and from a base stationusing the information on an added reference signal, and wherein theprocessor configured to set to allow the capacity information of theterminal to comprise a coverage improvement request level confirming anintensity of a downlink signal by the terminal and selectedcorresponding to the intensity of the signal, or information indicatingthat the terminal is a machine type communication (MTC) terminal, andconfirm that the added reference signal include at least one of newlydefined CRS, a demodulation (DM) reference signal, or a MBSFN-RS. 10.The apparatus of claim 9, wherein the processor configured to: receivethe added RS through a center resource block (RB) configured by resourceelements (RE) of a determined position of a physical downlink sharedchannel (PDSCH), and confirm that the RS comprises a selective powerboosting applied reference signal.
 11. The apparatus of claim 10,wherein the processor configured to receive a power boosted CRS throughsymbols except for symbols assigned for a physical downlink controlchannel (PDCCH) at a MBSFN sub-frame for transmitting a physicalmulticast channel (PMCH), and confirm that the CRS comprises data towhich QAM modulation is not applied.
 12. The apparatus of claim 9,wherein the processor configured to: receive an additional CRS from apredetermined region for a DM-RS, receive an additional CRS from secondand third symbols of each slot; or receive an additional CRS from asymbol determined according to at least one parameter distinguished froman initial CRS, and apply a V shift value indicating a group of theadditional CRS and an H shift value indicating a position a symbol of acorresponding group to the at least one parameter.
 13. The apparatus ofclaim 12, wherein the processor configured to: receive the added DM-RSat a predetermined changeable position according to the coverageenhancement request level, receive the added DM-RS in an RE of apredetermined DM-RS for a specific antenna port of a normal sub-frame,and receive the added DM-RS twice increased at a predetermined positionaccording to the coverage enhancement request level.
 14. The apparatusof claim 13, wherein the processor configured to: receive the addedDM-RS at the added DM-RS according to setting of a special sub-frame anda predetermined position of an extension sub-frame, and confirm that thedetermined position of the extension sub-frame is changed by slots byapplying a V shift value by taking into consideration collision with aCRS.
 15. The apparatus of claim 9, wherein the processor configured to:confirm that a transmission density is changeably set with respect to atleast one reference signal among an additional CRS, an additional DM-RS,an additional tracking reference signal (TRS), and a newly defined RS;and confirms that selective power boosting is applied to the at leastone reference signal.